diff --git a/vpi/vhdl_textio.c b/vpi/vhdl_textio.c
index 87a283090..5da433d39 100644
--- a/vpi/vhdl_textio.c
+++ b/vpi/vhdl_textio.c
@@ -521,7 +521,7 @@ static PLI_INT32 ivlh_readline_calltf(ICARUS_VPI_CONST PLI_BYTE8*name)
         return 0;
     }
 
-    int len = strlen(buf);
+    size_t len = strlen(buf);
 
     if(len == 0) {
         show_error_line(callh);