diff --git a/src/fix_external.h b/src/fix_external.h
index 9d4d8d1b62..35cd8deb03 100644
--- a/src/fix_external.h
+++ b/src/fix_external.h
@@ -41,7 +41,7 @@ class FixExternal : public Fix {
   int pack_exchange(int, double *);
   int unpack_exchange(int, double *);
 
-  typedef void (*FnPtr)(void *, int, int, int *, double **, double **);
+  typedef void (*FnPtr)(void *, bigint, int, int *, double **, double **);
   void set_callback(FnPtr, void *);
 
  private: