diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index c56c53e46..464809b27 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -835,7 +835,7 @@ mod pure_builtins {
     ) -> Result<Value, ErrorKind> {
         let beg = start.as_int()?;
         let len = len.as_int()?;
-        let x = s.to_str()?;
+        let x = s.coerce_to_string(co, CoercionKind::Weak).await?.to_str()?;
 
         if beg < 0 {
             return Err(ErrorKind::IndexOutOfBounds { index: beg });
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.exp
new file mode 100644
index 000000000..192548e94
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.exp
@@ -0,0 +1 @@
+"42"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.nix
new file mode 100644
index 000000000..626ae1d1b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.nix
@@ -0,0 +1,5 @@
+# builtins.substring uses string coercion internally
+
+builtins.substring 0 2 {
+  __toString = _: "4200";
+}