diff --git a/src/traces/heatmap/calc.js b/src/traces/heatmap/calc.js
index 32a82f76d3c..9b362500b05 100644
--- a/src/traces/heatmap/calc.js
+++ b/src/traces/heatmap/calc.js
@@ -67,10 +67,10 @@ module.exports = function calc(gd, trace) {
             y = trace.y ? ya.makeCalcdata(trace, 'y') : [];
         }
 
-        x0 = trace.x0 || 0;
-        dx = trace.dx || 1;
-        y0 = trace.y0 || 0;
-        dy = trace.dy || 1;
+        x0 = trace.x0;
+        dx = trace.dx;
+        y0 = trace.y0;
+        dy = trace.dy;
 
         z = clean2dArray(zIn, trace.transpose);
 
diff --git a/src/traces/heatmap/make_bound_array.js b/src/traces/heatmap/make_bound_array.js
index 84e09b1ce43..7abc1f6cd2e 100644
--- a/src/traces/heatmap/make_bound_array.js
+++ b/src/traces/heatmap/make_bound_array.js
@@ -63,20 +63,20 @@ module.exports = function makeBoundArray(trace, arrayIn, v0In, dvIn, numbricks,
         }
     }
     else {
-        dv = dvIn || 1;
-
         var calendar = trace[ax._id.charAt(0) + 'calendar'];
 
-        if(isHist || ax.type === 'category' || ax.type === 'multicategory') {
-            v0 = ax.r2c(v0In, 0, calendar) || 0;
-        } else if(isArrayOrTypedArray(arrayIn) && arrayIn.length === 1) {
+        if(isArrayOrTypedArray(arrayIn) && arrayIn.length === 1) {
             v0 = arrayIn[0];
         } else if(v0In === undefined) {
             v0 = 0;
+        } else if(isHist || ax.type === 'category' || ax.type === 'multicategory') {
+            v0 = ax.r2c(v0In, 0, calendar);
         } else {
             v0 = ax.d2c(v0In, 0, calendar);
         }
 
+        dv = dvIn || 1;
+
         for(i = (isContour || isGL2D) ? 0 : -0.5; i < numbricks; i++) {
             arrayOut.push(v0 + dv * i);
         }
diff --git a/test/image/baselines/heatmap-with-zero-category.png b/test/image/baselines/heatmap-with-zero-category.png
new file mode 100644
index 00000000000..765ce76bd2a
Binary files /dev/null and b/test/image/baselines/heatmap-with-zero-category.png differ
diff --git a/test/image/mocks/heatmap-with-zero-category.json b/test/image/mocks/heatmap-with-zero-category.json
new file mode 100644
index 00000000000..1280d125837
--- /dev/null
+++ b/test/image/mocks/heatmap-with-zero-category.json
@@ -0,0 +1,32 @@
+{
+  "data": [
+    {
+      "x": [ 3, 2, 1, 0 ],
+      "y": [ 3, 2, 1, 0 ],
+      "z": [
+        [ 98, 97, 96, null ],
+        [ 88, 87, 86, null ],
+        [ 78, 77, 76, null ],
+        [ null, null, null, null ]
+      ],
+      "type": "heatmap",
+      "colorscale": [
+        [ 0, "#3D9970" ],
+        [ 1, "#001f3f" ]
+      ],
+      "showscale": false,
+      "xgap": 1,
+      "ygap": 1
+    }
+  ],
+  "layout": {
+    "height": 450,
+    "width": 650,
+    "xaxis": {
+      "type": "category"
+    },
+    "yaxis": {
+      "type": "category"
+    }
+  }
+}
diff --git a/test/jasmine/tests/heatmap_test.js b/test/jasmine/tests/heatmap_test.js
index c6db00e0da5..0ae0005be82 100644
--- a/test/jasmine/tests/heatmap_test.js
+++ b/test/jasmine/tests/heatmap_test.js
@@ -287,10 +287,11 @@ describe('heatmap convertColumnXYZ', function() {
 describe('heatmap calc', function() {
     'use strict';
 
-    function _calc(opts) {
+    function _calc(opts, layout) {
         var base = { type: 'heatmap' };
         var trace = Lib.extendFlat({}, base, opts);
         var gd = { data: [trace] };
+        if(layout) gd.layout = layout;
 
         supplyAllDefaults(gd);
         var fullTrace = gd._fullData[0];
@@ -408,6 +409,20 @@ describe('heatmap calc', function() {
         expect(out.z).toBeCloseTo2DArray([[17, 18, 19]]);
     });
 
+    it('should handle the category case (edge case with a *0* category)', function() {
+        var out = _calc({
+            x: ['a', 'b', 0],
+            y: ['z', 0, 'y'],
+            z: [[17, 18, 19], [10, 20, 30], [40, 30, 20]]
+        }, {
+            xaxis: {type: 'category'},
+            yaxis: {type: 'category'}
+        });
+
+        expect(out.x).toBeCloseToArray([-0.5, 0.5, 1.5, 2.5]);
+        expect(out.y).toBeCloseToArray([-0.5, 0.5, 1.5, 2.5]);
+    });
+
     it('should handle the category x/y/z/ column case', function() {
         var out = _calc({
             x: ['a', 'a', 'a', 'b', 'b', 'b', 'c', 'c', 'c'],