diff --git a/fvid/fvid.py b/fvid/fvid.py
index f54b21c..1b75d61 100644
--- a/fvid/fvid.py
+++ b/fvid/fvid.py
@@ -219,8 +219,9 @@ def main():
         save_bits_to_file("./", bits)
 
     elif args.encode:
-        # isdigit has the benefit of raising an error if the user passes a negative string
-        if not args.framerate.isdigit() and "/" not in args.framerate:
+        # isdigit has the benefit of being True and raising an error if the user passes a negative string
+        # all() lets us check if both the negative sign and forward slash are in the string, to prevent negative fractions
+        if (not args.framerate.isdigit() and "/" not in args.framerate) or all(x in args.framerate for x in ("-", "/")):
             raise NotImplementedError("The framerate must be a positive fraction or an integer for now, like 3, '1/3', or '1/5'!")
         # get bits from file
         bits = get_bits_from_file(args.input)