diff --git a/ext/intl/locale/locale_methods.c b/ext/intl/locale/locale_methods.c
index c47f2832015..3471ba40539 100644
--- a/ext/intl/locale/locale_methods.c
+++ b/ext/intl/locale/locale_methods.c
@@ -1548,7 +1548,11 @@ PHP_FUNCTION(locale_lookup)
 	}
 
 	if(loc_range_len == 0) {
-		loc_range = intl_locale_get_default();
+		if(fallback_loc_str) {
+			loc_range = ZSTR_VAL(fallback_loc_str);
+		} else {
+			loc_range = intl_locale_get_default();
+		}
 	}
 
 	hash_arr = Z_ARRVAL_P(arr);
diff --git a/ext/intl/tests/locale_bug66289.phpt b/ext/intl/tests/locale_bug66289.phpt
new file mode 100644
index 00000000000..6afd821b5a8
--- /dev/null
+++ b/ext/intl/tests/locale_bug66289.phpt
@@ -0,0 +1,27 @@
+--TEST--
+Bug #66289 Locale::lookup incorrectly returns en or en_US if locale is empty
+--SKIPIF--
+<?php if( !extension_loaded( 'intl' ) ) print 'skip'; ?>
+--FILE--
+<?php
+
+ini_set("intl.default_locale", "en-US");
+$availableLocales = array('fr_FR', 'de', 'es_ES', 'es_419', 'en_US');
+var_dump(locale_lookup($availableLocales, false, true, 'fr_FR'));
+var_dump(locale_lookup($availableLocales, false, true, null));
+
+$availableLocales = array('fr_FR', 'de', 'es_ES', 'es_419');
+var_dump(locale_lookup($availableLocales, false, true, 'fr_FR'));
+
+ini_set("intl.default_locale", "de-DE");
+$availableLocales = array(Locale::getDefault());
+var_dump(locale_lookup($availableLocales, false, true));
+
+?>
+==DONE==
+--EXPECT--
+string(5) "fr_fr"
+string(5) "en_us"
+string(5) "fr_fr"
+string(5) "de_de"
+==DONE==