diff --git a/biome.json b/biome.json
index 4e1d7e8c0e..524a89bd51 100644
--- a/biome.json
+++ b/biome.json
@@ -23,6 +23,7 @@
         "noSwitchDeclarations": "off"
       },
       "nursery": {
+        "noSubstr": "error",
         "noUselessEscapeInRegex": "error"
       },
       "security": {
diff --git a/runtime/mlBytes.js b/runtime/mlBytes.js
index 577616952d..faf242546b 100644
--- a/runtime/mlBytes.js
+++ b/runtime/mlBytes.js
@@ -89,7 +89,6 @@ function caml_utf8_of_utf16(s) {
     if (c < 0x80) {
       for (var j = i + 1; j < l && (c = s.charCodeAt(j)) < 0x80; j++);
       if (j - i > 512) {
-        t.substr(0, 1);
         b += t;
         t = "";
         b += s.slice(i, j);
@@ -125,7 +124,6 @@ function caml_utf8_of_utf16(s) {
       );
     }
     if (t.length > 1024) {
-      t.substr(0, 1);
       b += t;
       t = "";
     }
@@ -140,7 +138,6 @@ function caml_utf16_of_utf8(s) {
     if (c1 < 0x80) {
       for (var j = i + 1; j < l && (c1 = s.charCodeAt(j)) < 0x80; j++);
       if (j - i > 512) {
-        t.substr(0, 1);
         b += t;
         t = "";
         b += s.slice(i, j);
@@ -179,7 +176,6 @@ function caml_utf16_of_utf8(s) {
       t += String.fromCharCode(0xd7c0 + (v >> 10), 0xdc00 + (v & 0x3ff));
     else t += String.fromCharCode(v);
     if (t.length > 1024) {
-      t.substr(0, 1);
       b += t;
       t = "";
     }