diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index fe5f05ab8..21dd5a294 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -3185,12 +3185,16 @@ std::ostream & operator << (std::ostream & str, const ExternalValueBase & v) {
     return v.print(str);
 }
 
-void forceNoNullByte(std::string_view s)
+void forceNoNullByte(std::string_view s, std::function<Pos()> pos)
 {
     if (s.find('\0') != s.npos) {
         using namespace std::string_view_literals;
         auto str = replaceStrings(std::string(s), "\0"sv, "␀"sv);
-        throw Error("input string '%s' cannot be represented as Nix string because it contains null bytes", str);
+        Error error("input string '%s' cannot be represented as Nix string because it contains null bytes", str);
+        if (pos) {
+            error.atPos(pos());
+        }
+        throw error;
     }
 }
 
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index a7e44cb72..067f86e01 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -41,16 +41,18 @@ namespace nix {
 
 // we make use of the fact that the parser receives a private copy of the input
 // string and can munge around in it.
-static StringToken unescapeStr(SymbolTable & symbols, char * s, size_t length)
+// getting the position is expensive and thus it is implemented lazily.
+static StringToken unescapeStr(char * const s, size_t length, std::function<Pos()> && pos)
 {
-    char * result = s;
+    bool noNullByte = true;
     char * t = s;
-    char c;
     // the input string is terminated with *two* NULs, so we can safely take
     // *one* character after the one being checked against.
-    while ((c = *s++)) {
+    for (size_t i = 0; i < length; t++) {
+        char c = s[i++];
+        noNullByte &= c != '\0';
         if (c == '\\') {
-            c = *s++;
+            c = s[i++];
             if (c == 'n') *t = '\n';
             else if (c == 'r') *t = '\r';
             else if (c == 't') *t = '\t';
@@ -59,12 +61,14 @@ static StringToken unescapeStr(SymbolTable & symbols, char * s, size_t length)
         else if (c == '\r') {
             /* Normalise CR and CR/LF into LF. */
             *t = '\n';
-            if (*s == '\n') s++; /* cr/lf */
+            if (s[i] == '\n') i++; /* cr/lf */
         }
         else *t = c;
-        t++;
     }
-    return {result, size_t(t - result)};
+    if (!noNullByte) {
+        forceNoNullByte({s, size_t(t - s)}, std::move(pos));
+    }
+    return {s, size_t(t - s)};
 }
 
 static void requireExperimentalFeature(const ExperimentalFeature & feature, const Pos & pos)
@@ -175,7 +179,7 @@ or          { return OR_KW; }
                 /* It is impossible to match strings ending with '$' with one
                    regex because trailing contexts are only valid at the end
                    of a rule. (A sane but undocumented limitation.) */
-                yylval->str = unescapeStr(state->symbols, yytext, yyleng);
+                yylval->str = unescapeStr(yytext, yyleng, [&]() { return state->positions[CUR_POS]; });
                 return STR;
               }
 <STRING>\$\{  { PUSH_STATE(DEFAULT); return DOLLAR_CURLY; }
@@ -191,6 +195,7 @@ or          { return OR_KW; }
 \'\'(\ *\n)?     { PUSH_STATE(IND_STRING); return IND_STRING_OPEN; }
 <IND_STRING>([^\$\']|\$[^\{\']|\'[^\'\$])+ {
                    yylval->str = {yytext, (size_t) yyleng, true};
+                   forceNoNullByte(yylval->str, [&]() { return state->positions[CUR_POS]; });
                    return IND_STR;
                  }
 <IND_STRING>\'\'\$ |
@@ -203,7 +208,7 @@ or          { return OR_KW; }
                    return IND_STR;
                  }
 <IND_STRING>\'\'\\{ANY} {
-                   yylval->str = unescapeStr(state->symbols, yytext + 2, yyleng - 2);
+                   yylval->str = unescapeStr(yytext + 2, yyleng - 2, [&]() { return state->positions[CUR_POS]; });
                    return IND_STR;
                  }
 <IND_STRING>\$\{ { PUSH_STATE(DEFAULT); return DOLLAR_CURLY; }
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 88fcae986..8925693e3 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -510,6 +510,6 @@ typedef std::shared_ptr<Value *> RootValue;
 
 RootValue allocRootValue(Value * v);
 
-void forceNoNullByte(std::string_view s);
+void forceNoNullByte(std::string_view s, std::function<Pos()> = nullptr);
 
 }
diff --git a/tests/functional/lang/eval-fail-string-nul-1.err.exp b/tests/functional/lang/eval-fail-string-nul-1.err.exp
new file mode 100644
index 000000000..2dfbea063
Binary files /dev/null and b/tests/functional/lang/eval-fail-string-nul-1.err.exp differ
diff --git a/tests/functional/lang/eval-fail-string-nul-1.nix b/tests/functional/lang/eval-fail-string-nul-1.nix
new file mode 100644
index 000000000..368940917
Binary files /dev/null and b/tests/functional/lang/eval-fail-string-nul-1.nix differ
diff --git a/tests/functional/lang/eval-fail-string-nul-2.err.exp b/tests/functional/lang/eval-fail-string-nul-2.err.exp
new file mode 100644
index 000000000..b1cae5325
Binary files /dev/null and b/tests/functional/lang/eval-fail-string-nul-2.err.exp differ
diff --git a/tests/functional/lang/eval-fail-string-nul-2.nix b/tests/functional/lang/eval-fail-string-nul-2.nix
new file mode 100644
index 000000000..fd6b3258a
Binary files /dev/null and b/tests/functional/lang/eval-fail-string-nul-2.nix differ