diff --git a/requirements.txt b/requirements.txt
index d8bcf54f..48cae7f7 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,9 +4,8 @@
 Cython
 matplotlib>=3.2.2
 numpy>=1.18.5
-opencv-python>=4.1.2
 Pillow
-PyYAML>=5.3.1
+PyYAML
 scipy>=1.4.1
 tensorboard>=1.5
 torch==1.7.0
diff --git a/utils/plots.py b/utils/plots.py
index c90a96b8..4a8f3836 100644
--- a/utils/plots.py
+++ b/utils/plots.py
@@ -160,7 +160,11 @@ def plot_images(images, targets, paths=None, fname='images.jpg', names=None, max
             for j, box in enumerate(boxes.T):
                 cls = int(classes[j])
                 color = colors[cls % len(colors)]
-                cls = names[cls] if names else cls
+                if names:
+                    if len(names) == 1: # when training 1 class detector, error occurs in here 
+                        cls = names[0]
+                    else:
+                        cls = names[cls]
                 if labels or conf[j] > 0.25:  # 0.25 conf thresh
                     label = '%s' % cls if labels else '%s %.1f' % (cls, conf[j])
                     plot_one_box(box, mosaic, label=label, color=color, line_thickness=tl)