diff --git a/rtengine/improcfun.cc b/rtengine/improcfun.cc
index 2e02a352c..79bd81c65 100644
--- a/rtengine/improcfun.cc
+++ b/rtengine/improcfun.cc
@@ -4778,13 +4778,22 @@ void ImProcFunctions::chromiLuminanceCurve(PipetteBuffer *pipetteBuffer, int pW,
                     histLCurve[Lprov1 * histLFactor]++;
                 }
 
-                Chprov1 = sqrt(SQR(atmp) + SQR(btmp)) / 327.68f;
 
                 // labCurve.bwtoning option allows to decouple modulation of a & b curves by saturation
                 // with bwtoning enabled the net effect of a & b curves is visible
                 if (bwToning) {
                     atmp -= lold->a[i][j];
                     btmp -= lold->b[i][j];
+                    Chprov1 = sqrt(SQR(atmp) + SQR(btmp)) / 327.68f;
+                    if (Chprov1 == 0.f) {
+                        sincosval.x = 0.f;
+                        sincosval.y = 1.f;
+                    } else {
+                        sincosval.x = btmp / (327.68f * Chprov1);
+                        sincosval.y = atmp / (327.68f * Chprov1);
+                    }
+                } else {
+                    Chprov1 = sqrt(SQR(atmp) + SQR(btmp)) / 327.68f;
                 }
 
                 lnew->L[i][j] = Lprov1 * 327.68f;