diff --git a/vdj_ann/src/vdj_features.rs b/vdj_ann/src/vdj_features.rs
index 579fac6d..d35260fb 100644
--- a/vdj_ann/src/vdj_features.rs
+++ b/vdj_ann/src/vdj_features.rs
@@ -466,9 +466,11 @@ pub fn fr3_start(aa: &[u8], chain_type: &str, verbose: bool) -> Option<usize> {
         ];
 
         // Score positions.
-
+        if cdr3_start < 42 - 2 {
+            return None;
+        }
         let mut score_pos = Vec::<(usize, isize)>::with_capacity(42 - 32 + 1);
-        for j in cdr3_start.saturating_sub(42 - 2)..=cdr3_start.saturating_sub(32 - 2) {
+        for j in cdr3_start - (42 - 2)..=cdr3_start - (32 - 2) {
             let mut score = 0;
             for (p, wm) in pwm.iter().enumerate() {
                 for l in wm {
@@ -525,7 +527,9 @@ pub fn fr3_start(aa: &[u8], chain_type: &str, verbose: bool) -> Option<usize> {
         ];
 
         // Score positions.
-
+        if cdr3_start < 36 {
+            return None;
+        }
         let mut score_pos = Vec::<(usize, usize)>::with_capacity(36 - 33 + 1);
         for j in cdr3_start - 36..=cdr3_start - 33 {
             let mut score = 0;
@@ -569,7 +573,9 @@ pub fn fr3_start(aa: &[u8], chain_type: &str, verbose: bool) -> Option<usize> {
         ];
 
         // Score positions.
-
+        if cdr3_start < 38 {
+            return None;
+        }
         let mut score_pos = Vec::<(usize, usize)>::with_capacity(38 - 35 + 1);
         for j in cdr3_start - 38..=cdr3_start - 35 {
             let mut score = 0;