diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 82a2ab831..86041cabd 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -4997,7 +4997,7 @@ bool Worker::pathContentsGood(const StorePath & path)
     if (!pathExists(store.printStorePath(path)))
         res = false;
     else {
-        HashResult current = hashPath(*info->narHash.type, store.printStorePath(path));
+        HashResult current = hashPath(info->narHash->type, store.printStorePath(path));
         Hash nullHash(htSHA256);
         res = info->narHash == nullHash || info->narHash == current.first;
     }
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index a05048290..de9b6a791 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -116,7 +116,8 @@ struct ValidPathInfo
 {
     StorePath path;
     std::optional<StorePath> deriver;
-    Hash narHash;
+    // TODO document this
+    std::optional<Hash> narHash;
     StorePathSet references;
     time_t registrationTime = 0;
     uint64_t narSize = 0; // 0 = unknown
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index e49eb4569..00bb999cb 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -16,16 +16,19 @@
 
 namespace nix {
 
+static size_t regularHashSize(HashType type) {
+    switch (type) {
+    case htMD5: return md5HashSize;
+    case htSHA1: return sha1HashSize;
+    case htSHA256: return sha256HashSize;
+    case htSHA512: return sha512HashSize;
+    }
+    abort();
+}
 
 void Hash::init()
 {
-    if (!type) abort();
-    switch (*type) {
-    case htMD5: hashSize = md5HashSize; break;
-    case htSHA1: hashSize = sha1HashSize; break;
-    case htSHA256: hashSize = sha256HashSize; break;
-    case htSHA512: hashSize = sha512HashSize; break;
-    }
+    hashSize = regularHashSize(type);
     assert(hashSize <= maxHashSize);
     memset(hash, 0, maxHashSize);
 }
@@ -105,18 +108,11 @@ string printHash16or32(const Hash & hash)
 }
 
 
-HashType assertInitHashType(const Hash & h) {
-    if (h.type)
-        return *h.type;
-    else
-        abort();
-}
-
 std::string Hash::to_string(Base base, bool includeType) const
 {
     std::string s;
     if (base == SRI || includeType) {
-        s += printHashType(assertInitHashType(*this));
+        s += printHashType(type);
         s += base == SRI ? '-' : ':';
     }
     switch (base) {
@@ -137,29 +133,36 @@ std::string Hash::to_string(Base base, bool includeType) const
 Hash::Hash(std::string_view s, HashType type) : Hash(s, std::optional { type }) { }
 Hash::Hash(std::string_view s) : Hash(s, std::optional<HashType>{}) { }
 
-Hash::Hash(std::string_view s, std::optional<HashType> type)
-    : type(type)
+Hash::Hash(std::string_view s, std::optional<HashType> optType)
 {
     size_t pos = 0;
     bool isSRI = false;
 
+    // Find the : or - separater, and set `isSRI` to the correct value
     auto sep = s.find(':');
     if (sep == string::npos) {
         sep = s.find('-');
-        if (sep != string::npos) {
+        if (sep != string::npos)
             isSRI = true;
-        } else if (! type)
-            throw BadHash("hash '%s' does not include a type", s);
     }
 
+    // Parse the has type before the separater, if there was one.
+    std::optional<HashType> optParsedType;
     if (sep != string::npos) {
-        string hts = string(s, 0, sep);
-        this->type = parseHashType(hts);
-        if (!this->type)
+        auto hts = s.substr(0, sep);
+        auto optParsedType = parseHashType(hts);
+        if (!optParsedType)
             throw BadHash("unknown hash type '%s'", hts);
-        if (type && type != this->type)
-            throw BadHash("hash '%s' should have type '%s'", s, printHashType(*type));
-        pos = sep + 1;
+    }
+
+    // Either the string or user must provide the type, if they both do they
+    // must agree.
+    if (!optParsedType && !optType) {
+        throw BadHash("hash '%s' does not include a type, nor is the type otherwise known from context.", s);
+    } else {
+        this->type = optParsedType ? *optParsedType : *optType;
+        if (optParsedType && optType && *optParsedType != *optType)
+            throw BadHash("hash '%s' should have type '%s'", s, printHashType(*optType));
     }
 
     init();
@@ -214,7 +217,7 @@ Hash::Hash(std::string_view s, std::optional<HashType> type)
     }
 
     else
-        throw BadHash("hash '%s' has wrong length for hash type '%s'", s, printHashType(*type));
+        throw BadHash("hash '%s' has wrong length for hash type '%s'", s, printHashType(type));
 }
 
 Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht)
@@ -267,7 +270,7 @@ static void finish(HashType ht, Ctx & ctx, unsigned char * hash)
 }
 
 
-Hash hashString(HashType ht, const string & s)
+Hash hashString(HashType ht, std::string_view s)
 {
     Ctx ctx;
     Hash hash(ht);
@@ -334,7 +337,7 @@ HashResult hashPath(
 
 Hash compressHash(const Hash & hash, unsigned int newSize)
 {
-    Hash h;
+    Hash h(hash.type);
     h.hashSize = newSize;
     for (unsigned int i = 0; i < hash.hashSize; ++i)
         h.hash[i % newSize] ^= hash.hash[i];
@@ -342,7 +345,7 @@ Hash compressHash(const Hash & hash, unsigned int newSize)
 }
 
 
-std::optional<HashType> parseHashTypeOpt(const string & s)
+std::optional<HashType> parseHashTypeOpt(std::string_view s)
 {
     if (s == "md5") return htMD5;
     else if (s == "sha1") return htSHA1;
@@ -351,7 +354,7 @@ std::optional<HashType> parseHashTypeOpt(const string & s)
     else return std::optional<HashType> {};
 }
 
-HashType parseHashType(const string & s)
+HashType parseHashType(std::string_view s)
 {
     auto opt_h = parseHashTypeOpt(s);
     if (opt_h)
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 0d9916508..b51850ccf 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -25,14 +25,11 @@ enum Base : int { Base64, Base32, Base16, SRI };
 
 struct Hash
 {
-    static const unsigned int maxHashSize = 64;
-    unsigned int hashSize = 0;
-    unsigned char hash[maxHashSize] = {};
+    constexpr static size_t maxHashSize = 64;
+    size_t hashSize = 0;
+    uint8_t hash[maxHashSize] = {};
 
-    std::optional<HashType> type = {};
-
-    /* Create an unset hash object. */
-    Hash() { };
+    HashType type;
 
     /* Create a zero-filled hash object. */
     Hash(HashType type) : type(type) { init(); };
@@ -105,7 +102,7 @@ Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht);
 string printHash16or32(const Hash & hash);
 
 /* Compute the hash of the given string. */
-Hash hashString(HashType ht, const string & s);
+Hash hashString(HashType ht, std::string_view s);
 
 /* Compute the hash of the given file. */
 Hash hashFile(HashType ht, const Path & path);
@@ -121,9 +118,9 @@ HashResult hashPath(HashType ht, const Path & path,
 Hash compressHash(const Hash & hash, unsigned int newSize);
 
 /* Parse a string representing a hash type. */
-HashType parseHashType(const string & s);
+HashType parseHashType(std::string_view s);
 /* Will return nothing on parse error */
-std::optional<HashType> parseHashTypeOpt(const string & s);
+std::optional<HashType> parseHashTypeOpt(std::string_view s);
 
 /* And the reverse. */
 string printHashType(HashType ht);
diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index 40b05a2f3..22410c44c 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -153,7 +153,7 @@ static int _main(int argc, char * * argv)
 
         /* If an expected hash is given, the file may already exist in
            the store. */
-        Hash hash, expectedHash(ht);
+        Hash hash(ht), expectedHash(ht);
         std::optional<StorePath> storePath;
         if (args.size() == 2) {
             expectedHash = Hash(args[1], ht);
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 4e02aa2bf..d061317ae 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -372,8 +372,8 @@ static void opQuery(Strings opFlags, Strings opArgs)
                 for (auto & j : maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise)) {
                     auto info = store->queryPathInfo(j);
                     if (query == qHash) {
-                        assert(info->narHash.type == htSHA256);
-                        cout << fmt("%s\n", info->narHash.to_string(Base32, true));
+                        assert(info->narHash && info->narHash->type == htSHA256);
+                        cout << fmt("%s\n", info->narHash->to_string(Base32, true));
                     } else if (query == qSize)
                         cout << fmt("%d\n", info->narSize);
                 }
@@ -725,7 +725,7 @@ static void opVerifyPath(Strings opFlags, Strings opArgs)
         auto path = store->followLinksToStorePath(i);
         printMsg(lvlTalkative, "checking path '%s'...", store->printStorePath(path));
         auto info = store->queryPathInfo(path);
-        HashSink sink(*info->narHash.type);
+        HashSink sink(info->narHash->type);
         store->narFromPath(path, sink);
         auto current = sink.finish();
         if (current.first != info->narHash) {
@@ -734,7 +734,7 @@ static void opVerifyPath(Strings opFlags, Strings opArgs)
                 .hint = hintfmt(
                     "path '%s' was modified! expected hash '%s', got '%s'",
                     store->printStorePath(path),
-                    info->narHash.to_string(Base32, true),
+                    info->narHash->to_string(Base32, true),
                     current.first.to_string(Base32, true))
             });
             status = 1;
@@ -864,7 +864,7 @@ static void opServe(Strings opFlags, Strings opArgs)
                         out << info->narSize // downloadSize
                             << info->narSize;
                         if (GET_PROTOCOL_MINOR(clientVersion) >= 4)
-                            out << (info->narHash ? info->narHash.to_string(Base32, true) : "") << info->ca << info->sigs;
+                            out << (info->narHash ? info->narHash->to_string(Base32, true) : "") << info->ca << info->sigs;
                     } catch (InvalidPath &) {
                     }
                 }
diff --git a/src/nix/add-to-store.cc b/src/nix/add-to-store.cc
index f43f774c1..0dda2af38 100644
--- a/src/nix/add-to-store.cc
+++ b/src/nix/add-to-store.cc
@@ -46,9 +46,9 @@ struct CmdAddToStore : MixDryRun, StoreCommand
         auto narHash = hashString(htSHA256, *sink.s);
 
         ValidPathInfo info(store->makeFixedOutputPath(FileIngestionMethod::Recursive, narHash, *namePart));
-        info.narHash = narHash;
+        *info.narHash = narHash;
         info.narSize = sink.s->size();
-        info.ca = makeFixedOutputCA(FileIngestionMethod::Recursive, info.narHash);
+        info.ca = makeFixedOutputCA(FileIngestionMethod::Recursive, *info.narHash);
 
         if (!dryRun) {
             auto source = StringSource { *sink.s };
diff --git a/src/nix/develop.cc b/src/nix/develop.cc
index 8b85caf82..4aee9f202 100644
--- a/src/nix/develop.cc
+++ b/src/nix/develop.cc
@@ -139,7 +139,7 @@ StorePath getDerivationEnvironment(ref<Store> store, const StorePath & drvPath)
         .path = shellOutPath,
         .hash = DerivationOutputHash {
             .method = FileIngestionMethod::Flat,
-            .hash = Hash { },
+            .hash = Hash { htSHA256 },
         },
     });
     drv.env["out"] = store->printStorePath(shellOutPath);