diff --git a/libs/prelude/Prelude/File.idr b/libs/prelude/Prelude/File.idr
index 5177e25c9..b60d4a2c2 100644
--- a/libs/prelude/Prelude/File.idr
+++ b/libs/prelude/Prelude/File.idr
@@ -150,7 +150,8 @@ do_getFileAccessTime h =
 private
 do_getFileModifiedTime : Ptr -> IO Integer
 do_getFileModifiedTime h = 
-   do MkRaw i <- foreign FFI_C "fileModifiedTime" (Ptr -> IO (Raw Integer)) h
+   do vm <- getMyVM
+      MkRaw i <- foreign FFI_C "fileModifiedTime" (Ptr -> Ptr -> IO (Raw Integer)) vm h
       pure i
 
 private
@@ -175,7 +176,7 @@ export
 fileModifiedTime : File -> IO (Either FileError Integer)
 fileModifiedTime (FHandle h)
     = do s <- do_getFileModifiedTime h
-         if (s < 0)
+         if (s == -1)
             then Left <$> getFileError
             else pure (Right s)
 
diff --git a/rts/idris_stdfgn.c b/rts/idris_stdfgn.c
index e1b9fc858..1507e9e8d 100644
--- a/rts/idris_stdfgn.c
+++ b/rts/idris_stdfgn.c
@@ -74,13 +74,13 @@ VAL fileAccessTime(void* h) {
     }
 }
 
-VAL fileModifiedTime(void* h) {
+VAL fileModifiedTime(VM* vm, void* h) {
     FILE* f = (FILE*)h;
     int fd = fileno(f);
 
     struct stat buf;
     if (fstat(fd, &buf) == 0) {
-        return MKBIGI(buf.st_mtime);
+        return MKBIGUI(vm, buf.st_mtime);
     } else {
         return MKBIGI(-1);
     }
diff --git a/rts/idris_stdfgn.h b/rts/idris_stdfgn.h
index 5342b981e..b0645991e 100644
--- a/rts/idris_stdfgn.h
+++ b/rts/idris_stdfgn.h
@@ -18,7 +18,7 @@ int fileSize(void* h);
 
 // Return a negative number if not a file (e.g. directory or device)
 VAL fileAccessTime(void* h);
-VAL fileModifiedTime(void* h);
+VAL fileModifiedTime(VM* vm, void* h);
 VAL fileStatusTime(void* h);
 
 void* idris_dirOpen(char* dname);