diff --git a/core/stdlib_ex.php b/core/stdlib_ex.php
index 0122d3fd..f2197f8f 100644
--- a/core/stdlib_ex.php
+++ b/core/stdlib_ex.php
@@ -5,10 +5,14 @@
  * @param T|false $x
  * @return T
  */
-function false_throws(mixed $x): mixed
+function false_throws(mixed $x, ?callable $errorgen=null): mixed
 {
     if($x === false) {
-        throw new \Exception("Unexpected false");
+        $msg = "Unexpected false";
+        if($errorgen) {
+            $msg = $errorgen();
+        }
+        throw new \Exception($msg);
     }
     return $x;
 }
@@ -18,10 +22,14 @@ function false_throws(mixed $x): mixed
  * @param T|null $x
  * @return T
  */
-function null_throws(mixed $x): mixed
+function null_throws(mixed $x, ?callable $errorgen=null): mixed
 {
     if($x === null) {
-        throw new \Exception("Unexpected null");
+        $msg = "Unexpected null";
+        if($errorgen) {
+            $msg = $errorgen();
+        }
+        throw new \Exception($msg);
     }
     return $x;
 }
@@ -31,7 +39,7 @@ function null_throws(mixed $x): mixed
  */
 function json_encode_ex(mixed $value, int|null $flags = 0, int $depth = 512): string
 {
-    return false_throws(json_encode($value, $flags, $depth));
+    return false_throws(json_encode($value, $flags, $depth), "json_last_error_msg");
 }
 
 function strtotime_ex(string $time, int|null $now = null): int
diff --git a/core/tests/StdLibExTest.php b/core/tests/StdLibExTest.php
new file mode 100644
index 00000000..4b2ee485
--- /dev/null
+++ b/core/tests/StdLibExTest.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Shimmie2;
+
+class StdLibExTest extends ShimmiePHPUnitTestCase
+{
+    public function testJsonEncodeOk(): void
+    {
+        $this->assertEquals(
+            '{"a":1,"b":2,"c":3,"d":4,"e":5}',
+            json_encode_ex(['a' => 1, 'b' => 2, 'c' => 3, 'd' => 4, 'e' => 5])
+        );
+    }
+
+    public function testJsonEncodeError(): void
+    {
+        $e = $this->assertException(\Exception::class, function() {
+            json_encode_ex("\xB1\x31");
+        });
+        $this->assertEquals(
+            "Malformed UTF-8 characters, possibly incorrectly encoded",
+            $e->getMessage()
+        );
+    }
+}