diff --git a/src/read_sequencing.py b/src/read_sequencing.py
index 2382f5166cc9b0512e77d2084ff1dbd59189634e..dfd7da99999727ce48b87ad7403a26a8b9ddf36c 100644
--- a/src/read_sequencing.py
+++ b/src/read_sequencing.py
@@ -82,7 +82,7 @@ def read_sequencing(
                 if len(frag) < read_len:
 
                     # Calculate number of random nucleotides to add to the end of the read
-                    diff = read_len - read_len
+                    diff = read_len - len(frag)
 
                     # Select random nucleotides from list of possible
                     rand_samp = choices(nucleotides, k=diff)