diff --git a/ext/curl/interface.c b/ext/curl/interface.c
index c0286069776e..c3001696483a 100644
--- a/ext/curl/interface.c
+++ b/ext/curl/interface.c
@@ -820,6 +820,17 @@ static size_t curl_read(char *data, size_t size, size_t nmemb, void *ctx)
 					memcpy(data, Z_STRVAL(retval), length);
 				} else if (Z_TYPE(retval) == IS_LONG) {
 					length = Z_LVAL_P(&retval);
+
+					switch (length) {
+						// Acceptable long values:
+						case 0:
+						case CURL_READFUNC_ABORT:
+						case CURL_READFUNC_PAUSE:
+							break;
+						default:
+							zend_value_error("The CURLOPT_READFUNCTION callback must return a string or CURL_READFUNC_ABORT or CURL_READFUNC_PAUSE");
+							length = CURL_READFUNC_ABORT;
+					}
 				}
 				// TODO Do type error if invalid type?
 				zval_ptr_dtor(&retval);
diff --git a/ext/curl/tests/curl_read_function_error_on_int.phpt b/ext/curl/tests/curl_read_function_error_on_int.phpt
new file mode 100644
index 000000000000..30ba97737727
--- /dev/null
+++ b/ext/curl/tests/curl_read_function_error_on_int.phpt
@@ -0,0 +1,30 @@
+--TEST--
+error when CURLOPT_READFUNCTION returns an integer
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+function custom_readfunction($oCurl, $hReadHandle, $iMaxOut)
+{
+    static $size = 2;
+    return $size--;
+}
+
+include 'server.inc';
+$host = curl_cli_server_start();
+$ch = curl_init();
+curl_setopt($ch, CURLOPT_URL, "{$host}/get.php?test=post");
+curl_setopt($ch, CURLOPT_POST, ['f' => 'f']);
+curl_setopt($ch, CURLOPT_TIMEOUT, 2);
+curl_setopt($ch, CURLOPT_READFUNCTION, "custom_readfunction" );
+
+try {
+    curl_exec($ch);
+} catch (ValueError $e) {
+    echo $e->getMessage() . "\n";
+}
+var_dump(curl_error($ch));
+?>
+--EXPECT--
+The CURLOPT_READFUNCTION callback must return a string or CURL_READFUNC_ABORT or CURL_READFUNC_PAUSE
+string(29) "operation aborted by callback"