diff --git a/src/if_py_both.h b/src/if_py_both.h
index 1a4ef46..1c15926 100644
--- a/src/if_py_both.h
+++ b/src/if_py_both.h
@@ -2953,7 +2953,7 @@
 	    char_u *np;
 	    size_t len = STRLEN(p) + 1;
 
-	    if ((np = alloc(len + 2)) == NULL)
+	    if ((np = alloc((int)len + 2)) == NULL)
 	    {
 		vim_free(p);
 		return NULL;