diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 6778023f5..c88f677da 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1601,12 +1601,16 @@ static void prim_partition(EvalState & state, const Pos & pos, Value * * args, V
     state.mkAttrs(v, 2);
 
     Value * vRight = state.allocAttr(v, state.sRight);
-    state.mkList(*vRight, right.size());
-    memcpy(vRight->listElems(), right.data(), sizeof(Value *) * right.size());
+    auto rsize = right.size();
+    state.mkList(*vRight, rsize);
+    if (rsize)
+        memcpy(vRight->listElems(), right.data(), sizeof(Value *) * rsize);
 
     Value * vWrong = state.allocAttr(v, state.sWrong);
-    state.mkList(*vWrong, wrong.size());
-    memcpy(vWrong->listElems(), wrong.data(), sizeof(Value *) * wrong.size());
+    auto wsize = wrong.size();
+    state.mkList(*vWrong, wsize);
+    if (wsize)
+        memcpy(vWrong->listElems(), wrong.data(), sizeof(Value *) * wsize);
 
     v.attrs->sort();
 }
diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index 5ab625f42..9d9f1153f 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -195,6 +195,7 @@ struct CurlDownloader : public Downloader
             if (readOffset == request.data->length())
                 return 0;
             auto count = std::min(size * nitems, request.data->length() - readOffset);
+            assert(count);
             memcpy(buffer, request.data->data() + readOffset, count);
             readOffset += count;
             return count;
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 75e476755..150995f55 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -191,6 +191,7 @@ Hash::Hash(const std::string & s, HashType type)
         auto d = base64Decode(std::string(s, pos));
         if (d.size() != hashSize)
             throw BadHash("invalid base-64 hash '%s'", s);
+        assert(hashSize);
         memcpy(hash, d.data(), hashSize);
     }