diff --git a/src/json.c b/src/json.c
index e1f40bf..cc75648 100644
--- a/src/json.c
+++ b/src/json.c
@@ -217,7 +217,7 @@
 
 	case VAR_NUMBER:
 	    vim_snprintf((char *)numbuf, NUMBUFLEN, "%lld",
-						    val->vval.v_number);
+						(long long)val->vval.v_number);
 	    ga_concat(gap, numbuf);
 	    break;