diff --git a/libs/fileio/fileio.c b/libs/fileio/fileio.c
index 63651ed..d888617 100644
--- a/libs/fileio/fileio.c
+++ b/libs/fileio/fileio.c
@@ -78,7 +78,7 @@ GroundValue fileStructRead(GroundScope* scope, List args) {
     if (sizeField == NULL) {
         ERROR("A field called \"size\" was not found", "FieldNotFound");
     }
-    long size = sizeField->value.data.intVal;
+    int64_t size = sizeField->value.data.intVal;
 
     // allocate buffer to read file into
     char* buffer = malloc(size + 1);