diff --git a/modules/gmt/modgmt_internals.h b/modules/gmt/modgmt_internals.h
index e8611cc..4b33dd9 100644
--- a/modules/gmt/modgmt_internals.h
+++ b/modules/gmt/modgmt_internals.h
@@ -33,7 +33,8 @@ inline struct GMT_OPTION* str2options(void *api, const char* str, size_t size=0)
  else
  {
   t=static_cast<char*>(malloc(size+1));
-  memcpy(t,str,size+1);
+  memcpy(t,str,size);
+  t[size]=0;
  }
  struct GMT_OPTION* opts=GMT_Create_Options(api,0,t);
  GMT_Append_Option(api,GMT_Create_Options(api,0,default_gmt_options),opts);