https://github.com/mkoppanen/imagick/issues/264

--- a/imagick_file.c
+++ b/imagick_file.c
@@ -160,11 +160,13 @@ static
 int php_imagick_read_image_using_imagemagick(php_imagick_object *intern, struct php_imagick_file_t *file, ImagickOperationType type TSRMLS_DC)
 {
 
+#ifdef ZTS
 #ifndef ZEND_ENGINE_3
 #if PHP_VERSION_ID >= 50600
 	// This suppresses an 'unused parameter' warning.
 	(void)tsrm_ls;
 #endif
+#endif
 #endif
 
 	if (type == ImagickReadImage) {