diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index e6314f63e..39f1a625f 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -934,7 +934,7 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 void EvalState::mkPos(Value & v, PosIdx p)
 {
     auto pos = positions[p];
-    if (pos.file.set()) {
+    if (!pos.file.empty()) {
         auto attrs = buildBindings(3);
         attrs.alloc(sFile).mkString(pos.file);
         attrs.alloc(sLine).mkInt(pos.line);
@@ -1296,7 +1296,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
 
     } catch (Error & e) {
         auto pos2r = state.positions[pos2];
-        if (pos2 && pos2r.file != state.sDerivationNix)
+        if (pos2 && pos2r.file != state.derivationNixPath)
             state.addErrorTrace(e, pos2, "while evaluating the attribute '%1%'",
                 showAttrPath(state, env, attrPath));
         throw;
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index b05e8d5d0..71d6e7e7f 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -75,6 +75,8 @@ public:
     SymbolTable symbols;
     PosTable positions;
 
+    static inline std::string derivationNixPath = "//builtin/derivation.nix";
+
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index d9392cff5..2e12f0b4f 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -26,7 +26,7 @@ MakeError(RestrictedPathError, Error);
 
 struct Pos
 {
-    Symbol file;
+    std::string file;
     FileOrigin origin;
     uint32_t line;
     uint32_t column;
@@ -64,10 +64,10 @@ public:
         explicit Origin(uint32_t idx): idx(idx), file{}, origin{} {}
 
     public:
-        const Symbol file;
+        const std::string file;
         const FileOrigin origin;
 
-        Origin(Symbol file, FileOrigin origin): file(file), origin(origin) {}
+        Origin(std::string file, FileOrigin origin): file(std::move(file)), origin(origin) {}
     };
 
     struct Offset {
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 0052a8070..1f950a057 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -648,14 +648,14 @@ Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
     const PathView path, const PathView basePath, StaticEnv & staticEnv)
 {
     yyscan_t scanner;
-    Symbol file;
+    std::string file;
     switch (origin) {
         case foFile:
-            file = symbols.create(path);
+            file = path;
             break;
         case foStdin:
         case foString:
-            file = symbols.create(text);
+            file = text;
             break;
         default:
             assert(false);
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 9cdcfd0b9..81fb5acfb 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -3915,7 +3915,7 @@ void EvalState::createBaseEnv()
 
     /* Add a wrapper around the derivation primop that computes the
        `drvPath' and `outPath' attributes lazily. */
-    sDerivationNix = symbols.create("//builtin/derivation.nix");
+    sDerivationNix = symbols.create(derivationNixPath);
     auto vDerivation = allocValue();
     addConstant("derivation", vDerivation);
 
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 348018f57..f4706e3ed 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -87,11 +87,7 @@ struct ErrPos {
         origin = pos.origin;
         line = pos.line;
         column = pos.column;
-        // is file symbol null?
-        if (pos.file.set())
-            file = pos.file;
-        else
-            file = "";
+        file = pos.file;
         return *this;
     }