diff --git a/compiler/can/src/builtins.rs b/compiler/can/src/builtins.rs
index 6b85c8343d..6cf4146ab0 100644
--- a/compiler/can/src/builtins.rs
+++ b/compiler/can/src/builtins.rs
@@ -1557,6 +1557,7 @@ fn str_from_utf8_range(symbol: Symbol, var_store: &mut VarStore) -> Def {
 
     let start_var = var_store.fresh();
     let start_bool = var_store.fresh();
+    let add_var = var_store.fresh();
 
     let body = If {
         cond_var: start_bool,
@@ -1567,12 +1568,31 @@ fn str_from_utf8_range(symbol: Symbol, var_store: &mut VarStore) -> Def {
                 args: vec![
                     (
                         start_var,
-                        Access {
-                            record_var: arg_record_var,
-                            ext_var: var_store.fresh(),
-                            field: "start".into(),
-                            field_var: var_store.fresh(),
-                            loc_expr: Box::new(no_region(Var(Symbol::ARG_2))),
+                        RunLowLevel {
+                            op: LowLevel::NumAdd,
+                            args: vec![
+                                (
+                                    add_var,
+                                    Access {
+                                        record_var: arg_record_var,
+                                        ext_var: var_store.fresh(),
+                                        field: "start".into(),
+                                        field_var: var_store.fresh(),
+                                        loc_expr: Box::new(no_region(Var(Symbol::ARG_2))),
+                                    },
+                                ),
+                                (
+                                    add_var,
+                                    Access {
+                                        record_var: arg_record_var,
+                                        ext_var: var_store.fresh(),
+                                        field: "count".into(),
+                                        field_var: var_store.fresh(),
+                                        loc_expr: Box::new(no_region(Var(Symbol::ARG_2))),
+                                    },
+                                ),
+                            ],
+                            ret_var: add_var,
                         },
                     ),
                     (
diff --git a/compiler/test_gen/src/gen_str.rs b/compiler/test_gen/src/gen_str.rs
index 4cbb2d9942..e0978817f9 100644
--- a/compiler/test_gen/src/gen_str.rs
+++ b/compiler/test_gen/src/gen_str.rs
@@ -896,7 +896,7 @@ fn str_from_utf8_range_order_does_not_matter() {
 }
 
 #[test]
-fn str_from_utf8_range_out_of_bounds() {
+fn str_from_utf8_range_out_of_bounds_start_value() {
     assert_evals_to!(
         indoc!(
             r#"
@@ -911,3 +911,37 @@ fn str_from_utf8_range_out_of_bounds() {
         RocStr
     );
 }
+
+#[test]
+fn str_from_utf8_range_count_too_high() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            bytes = Str.toUtf8 "hello"
+            when Str.fromUtf8Range bytes { start: 0,  count: 6 }  is
+                   Ok _ -> ""
+                   Err (BadUtf8 _ _) -> ""
+                   Err OutOfBounds -> "out of bounds"
+            "#
+        ),
+        RocStr::from("out of bounds"),
+        RocStr
+    );
+}
+
+#[test]
+fn str_from_utf8_range_count_too_high_for_start() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            bytes = Str.toUtf8 "hello"
+            when Str.fromUtf8Range bytes { start: 4,  count: 3 }  is
+                   Ok _ -> ""
+                   Err (BadUtf8 _ _) -> ""
+                   Err OutOfBounds -> "out of bounds"
+            "#
+        ),
+        RocStr::from("out of bounds"),
+        RocStr
+    );
+}