diff --git a/src/fwrap.cpp b/src/fwrap.cpp
index 1aa545f4..43d81ba9 100644
--- a/src/fwrap.cpp
+++ b/src/fwrap.cpp
@@ -25,7 +25,7 @@ f2cstring(char* fstring, size_t len)
 }
 
 void
-padfstring(char *dest, const char *src, size_t len)
+padfstring(char *dest, const char *src, unsigned int len)
 {
     size_t sofar;
 
diff --git a/src/fwrap.h b/src/fwrap.h
index c55ad736..49a33da1 100644
--- a/src/fwrap.h
+++ b/src/fwrap.h
@@ -153,5 +153,5 @@ extern "C" {
 #if defined(__cplusplus)
 }
 #endif
-void padfstring(char *dest, const char *src, size_t len);
+void padfstring(char *dest, const char *src, unsigned int len);
 #endif  /* __FWRAP__H */