diff --git a/nix/tools.nix b/nix/tools.nix
index 108d3860..47e99450 100644
--- a/nix/tools.nix
+++ b/nix/tools.nix
@@ -9,7 +9,12 @@ in {
     let
       linesFrom = file: splitString "\n" (fileContents file);
       matching = regex: lines: map (line: match regex line) lines;
-      extractMatch = matches: last (flatten (remove null matches));
+      extractMatch = matches:
+        let xs = flatten (remove null matches);
+        in if xs == [] then
+          throw "findKeyValue: no match for regex '${regex}' in ${toString sourceFile}"
+        else
+          last xs;
     in
       extractMatch (matching regex (linesFrom sourceFile));
 }