diff --git a/libavfilter/vf_curves.c b/libavfilter/vf_curves.c
index 4f302209e6..8ff69807d9 100644
--- a/libavfilter/vf_curves.c
+++ b/libavfilter/vf_curves.c
@@ -813,11 +813,11 @@ static int filter_slice_planar(AVFilterContext *ctx, void *arg, int jobnr, int n
             uint16_t       *dstrp = (      uint16_t *)(out->data[r] + y * out->linesize[r]);
             uint16_t       *dstgp = (      uint16_t *)(out->data[g] + y * out->linesize[g]);
             uint16_t       *dstbp = (      uint16_t *)(out->data[b] + y * out->linesize[b]);
-            uint16_t       *dstap = (      uint16_t *)(out->data[a] + y * out->linesize[a]);
+            uint16_t       *dstap = (      uint16_t *)(step == 4 ? out->data[a] + y * out->linesize[a] : NULL);
             const uint16_t *srcrp = (const uint16_t *)(in ->data[r] + y *  in->linesize[r]);
             const uint16_t *srcgp = (const uint16_t *)(in ->data[g] + y *  in->linesize[g]);
             const uint16_t *srcbp = (const uint16_t *)(in ->data[b] + y *  in->linesize[b]);
-            const uint16_t *srcap = (const uint16_t *)(in ->data[a] + y *  in->linesize[a]);
+            const uint16_t *srcap = (const uint16_t *)(step == 4 ? in ->data[a] + y *  in->linesize[a] : NULL);
 
             for (x = 0; x < in->width; x++) {
                 dstrp[x] = curves->graph[R][srcrp[x]];
@@ -831,11 +831,13 @@ static int filter_slice_planar(AVFilterContext *ctx, void *arg, int jobnr, int n
         uint8_t       *dstr = out->data[r] + slice_start * out->linesize[r];
         uint8_t       *dstg = out->data[g] + slice_start * out->linesize[g];
         uint8_t       *dstb = out->data[b] + slice_start * out->linesize[b];
-        uint8_t       *dsta = out->data[a] + slice_start * out->linesize[a];
+        uint8_t       *dsta =    step == 4 ? out->data[a]
+                                           + slice_start * out->linesize[a] : NULL;
         const uint8_t *srcr =  in->data[r] + slice_start *  in->linesize[r];
         const uint8_t *srcg =  in->data[g] + slice_start *  in->linesize[g];
         const uint8_t *srcb =  in->data[b] + slice_start *  in->linesize[b];
-        const uint8_t *srca =  in->data[a] + slice_start *  in->linesize[a];
+        const uint8_t *srca =    step == 4 ?  in->data[a]
+                                           + slice_start *  in->linesize[a] : NULL;
 
         for (y = slice_start; y < slice_end; y++) {
             for (x = 0; x < in->width; x++) {
@@ -848,11 +850,13 @@ static int filter_slice_planar(AVFilterContext *ctx, void *arg, int jobnr, int n
             dstr += out->linesize[r];
             dstg += out->linesize[g];
             dstb += out->linesize[b];
-            dsta += out->linesize[a];
             srcr += in ->linesize[r];
             srcg += in ->linesize[g];
             srcb += in ->linesize[b];
-            srca += in ->linesize[a];
+            if (step == 4) {
+                dsta += out->linesize[a];
+                srca += in ->linesize[a];
+            }
         }
     }
     return 0;