diff --git a/data/shaders/stroke.glsl b/data/shaders/stroke.glsl
index d9c6a29..cd0d61a 100644
--- a/data/shaders/stroke.glsl
+++ b/data/shaders/stroke.glsl
@@ -75,7 +75,7 @@ void main()
             patt = brightness1(patt, 1.0 - pattern_bright);
         if (pattern_contr != 0.5)
             patt = contrast1(patt, pattern_contr);
-        fg.a = blend_stroke(fg.a, patt, pattern_depth, patt_blend_mode);
+        fg.a = clamp(blend_stroke(fg.a, patt, pattern_depth, patt_blend_mode), 0.0, 1.0);
     }
 
 #if defined(GL_EXT_shader_framebuffer_fetch)