diff --git a/caps/src/nsBasePrincipal.cpp b/caps/src/nsBasePrincipal.cpp
index d63b832279e0..d72821507b55 100644
--- a/caps/src/nsBasePrincipal.cpp
+++ b/caps/src/nsBasePrincipal.cpp
@@ -61,7 +61,7 @@ nsBasePrincipal::CanEnableCapability(const char *capability, PRInt16 *result)
         return NS_OK;
     }
     nsStringKey key(capability);
-    *result = (PRInt16) mCapabilities->Get(&key);
+    *result = (PRInt16)(PRInt32)mCapabilities->Get(&key);
     if (!*result)
         *result = nsIPrincipal::ENABLE_UNKNOWN;
     return NS_OK;