diff --git a/src/unix/mimetype.cpp b/src/unix/mimetype.cpp
index f9c9d23857..f5e986fe99 100644
--- a/src/unix/mimetype.cpp
+++ b/src/unix/mimetype.cpp
@@ -124,6 +124,11 @@ public:
             if ( idx )
                 *idx = n;
         }
+        else if ( idx )
+        {
+            // different from any valid index
+            *idx = (size_t)-1;
+        }
 
         return s;
     }