--- a/metric-providers/org.ossmeter.metricprovider.rascal/modules/Manager.rsc
+++ b/metric-providers/org.ossmeter.metricprovider.rascal/modules/Manager.rsc
@@ -34,7 +34,9 @@
 
     return m3WithAdditionalInfo(fileM3, createAstFromFile(file, true), total, comment, empty, source);
   }
-  return unknownFileType(size(readFileLines(file)));
+  if (isFile(file))
+    return unknownFileType(size(readFileLines(file)));
+  throw "Location <file> is not a file";
 }
 
 bool isValid(M3 fileM3) {