diff --git a/intern/cycles/render/light.cpp b/intern/cycles/render/light.cpp
index 2245c861d5a..6a4557506c3 100644
--- a/intern/cycles/render/light.cpp
+++ b/intern/cycles/render/light.cpp
@@ -486,10 +486,18 @@ static void background_cdf(int start,
                            float2 *cond_cdf)
 {
 	/* Conditional CDFs (rows, U direction). */
+	/* NOTE: It is possible to have some NaN pixels on background
+	 * which will ruin CDF causing wrong shading. We replace such
+	 * pixels with black.
+	 */
 	for(int i = start; i < end; i++) {
 		float sin_theta = sinf(M_PI_F * (i + 0.5f) / res);
 		float3 env_color = (*pixels)[i * res];
 		float ave_luminance = average(env_color);
+		/* TODO(sergey): Consider adding average_safe(). */
+		if(!isfinite(ave_luminance)) {
+			ave_luminance = 0.0f;
+		}
 
 		cond_cdf[i * cdf_count].x = ave_luminance * sin_theta;
 		cond_cdf[i * cdf_count].y = 0.0f;
@@ -497,6 +505,9 @@ static void background_cdf(int start,
 		for(int j = 1; j < res; j++) {
 			env_color = (*pixels)[i * res + j];
 			ave_luminance = average(env_color);
+			if(!isfinite(ave_luminance)) {
+				ave_luminance = 0.0f;
+			}
 
 			cond_cdf[i * cdf_count + j].x = ave_luminance * sin_theta;
 			cond_cdf[i * cdf_count + j].y = cond_cdf[i * cdf_count + j - 1].y + cond_cdf[i * cdf_count + j - 1].x / res;