diff --git a/src/libexpr/paths.cc b/src/libexpr/paths.cc
index 826a738a6..da1408e9b 100644
--- a/src/libexpr/paths.cc
+++ b/src/libexpr/paths.cc
@@ -83,8 +83,16 @@ StorePath EvalState::mountInput(
     }
 
     // FIXME: what to do with the NAR hash in lazy mode?
-    if (!settings.lazyTrees)
-        assert(!originalInput.getNarHash() || storePath == originalInput.computeStorePath(*store));
+    if (!settings.lazyTrees && originalInput.getNarHash()) {
+        auto expected = originalInput.computeStorePath(*store);
+        if (storePath != expected)
+            throw Error(
+                (unsigned int) 102,
+                "NAR hash mismatch in input '%s', expected '%s' but got '%s'",
+                originalInput.to_string(),
+                store->printStorePath(storePath),
+                store->printStorePath(expected));
+    }
 
     return storePath;
 }
diff --git a/tests/functional/fetchGit.sh b/tests/functional/fetchGit.sh
index baa09b60b..6fc8ca8b0 100755
--- a/tests/functional/fetchGit.sh
+++ b/tests/functional/fetchGit.sh
@@ -142,14 +142,13 @@ path4=$(nix eval --impure --refresh --raw --expr "(builtins.fetchGit file://$rep
 [[ $(nix eval --impure --expr "builtins.hasAttr \"dirtyRev\" (builtins.fetchGit $repo)") == "false" ]]
 [[ $(nix eval --impure --expr "builtins.hasAttr \"dirtyShortRev\" (builtins.fetchGit $repo)") == "false" ]]
 
-# FIXME: check narHash
-#expect 102 nix eval --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-B5yIPHhEm0eysJKEsO7nqxprh9vcblFxpJG11gXJus1=\"; }).outPath"
+expect 102 nix eval --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-B5yIPHhEm0eysJKEsO7nqxprh9vcblFxpJG11gXJus1=\"; }).outPath"
 
 path5=$(nix eval --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-Hr8g6AqANb3xqX28eu1XnjK/3ab8Gv6TJSnkb1LezG9=\"; }).outPath")
 [[ $path = $path5 ]]
 
 # Ensure that NAR hashes are checked.
-#expectStderr 102 nix eval --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-Hr8g6AqANb4xqX28eu1XnjK/3ab8Gv6TJSnkb1LezG9=\"; }).outPath" | grepQuiet "error: NAR hash mismatch"
+expectStderr 102 nix eval --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-Hr8g6AqANb4xqX28eu1XnjK/3ab8Gv6TJSnkb1LezG9=\"; }).outPath" | grepQuiet "error: NAR hash mismatch"
 
 # It's allowed to use only a narHash, but you should get a warning.
 expectStderr 0 nix eval --raw --expr "(builtins.fetchGit { url = $repo; ref = \"tag2\"; narHash = \"sha256-Hr8g6AqANb3xqX28eu1XnjK/3ab8Gv6TJSnkb1LezG9=\"; }).outPath" | grepQuiet "warning: Input .* is unlocked"