diff --git a/src/qrcode/cpp_port/detector.rs b/src/qrcode/cpp_port/detector.rs
index 1463a7a..37250cc 100644
--- a/src/qrcode/cpp_port/detector.rs
+++ b/src/qrcode/cpp_port/detector.rs
@@ -240,6 +240,7 @@ pub fn GenerateFinderPatternSets(patterns: &mut FinderPatterns) -> FinderPattern
 
 pub fn EstimateModuleSize(image: &BitMatrix, a: ConcentricPattern, b: ConcentricPattern) -> f64 {
     let mut cur = EdgeTracer::new(image, a.p, b.p - a.p);
+    if !cur.isBlack() { return  -1.0; }
     assert!(cur.isBlack());
 
     let pattern = ReadSymmetricPattern::<5, _>(&mut cur, a.size * 2);
diff --git a/tests/cpp_qr_code_blackbox_tests.rs b/tests/cpp_qr_code_blackbox_tests.rs
index 837963e..519643b 100644
--- a/tests/cpp_qr_code_blackbox_tests.rs
+++ b/tests/cpp_qr_code_blackbox_tests.rs
@@ -15,9 +15,8 @@
  */
 
 use rxing::{
-    multi::MultipleBarcodeReader,
-    qrcode::{cpp_port::QrReader, QRCodeReader},
-    BarcodeFormat, MultiFormatReader,
+    qrcode::{cpp_port::QrReader},
+    BarcodeFormat,
 };
 
 mod common;