diff --git a/libraries/AP_Common/Bitmask.h b/libraries/AP_Common/Bitmask.h
index 856257d468..70870c6271 100644
--- a/libraries/AP_Common/Bitmask.h
+++ b/libraries/AP_Common/Bitmask.h
@@ -108,7 +108,15 @@ public:
             if (bits[i] == 0) {
                 continue;
             }
-            return i*32 + __builtin_ffs(bits[i]) - 1;
+            int fs;
+            if (sizeof(bits[i]) <= sizeof(int)) {
+                fs = __builtin_ffs(bits[i]);
+            } else if (sizeof(bits[i]) <= sizeof(long)) {
+                fs = __builtin_ffsl(bits[i]);
+            } else {
+                fs = __builtin_ffsll(bits[i]);
+            }
+            return i*32 + fs - 1;
         }
         return -1;
     }