diff --git a/src/reducers/brush-tool.js b/src/reducers/brush-tool.js
index 0bc422ce..7faca7f6 100644
--- a/src/reducers/brush-tool.js
+++ b/src/reducers/brush-tool.js
@@ -1,3 +1,5 @@
+import log from '../log/log';
+
 const CHANGE_BRUSH_SIZE = 'scratch-paint/tools/CHANGE_BRUSH_SIZE';
 const initialState = {brushSize: 5};
 
@@ -5,6 +7,10 @@ const reducer = function (state, action) {
     if (typeof state === 'undefined') state = initialState;
     switch (action.type) {
     case CHANGE_BRUSH_SIZE:
+        if (isNaN(action.brushSize)) {
+            log.warn(`Invalid brush size: ${action.brushSize}`);
+            return state;
+        }
         return {brushSize: Math.max(1, action.brushSize)};
     default:
         return state;
diff --git a/src/reducers/eraser-tool.js b/src/reducers/eraser-tool.js
index a985e666..50000eb4 100644
--- a/src/reducers/eraser-tool.js
+++ b/src/reducers/eraser-tool.js
@@ -1,3 +1,5 @@
+import log from '../log/log';
+
 const CHANGE_ERASER_SIZE = 'scratch-paint/tools/CHANGE_ERASER_SIZE';
 const initialState = {brushSize: 20};
 
@@ -5,6 +7,10 @@ const reducer = function (state, action) {
     if (typeof state === 'undefined') state = initialState;
     switch (action.type) {
     case CHANGE_ERASER_SIZE:
+        if (isNaN(action.brushSize)) {
+            log.warn(`Invalid brush size: ${action.brushSize}`);
+            return state;
+        }
         return {brushSize: Math.max(1, action.brushSize)};
     default:
         return state;
diff --git a/test/unit/blob-tool-reducer.test.js b/test/unit/blob-tool-reducer.test.js
new file mode 100644
index 00000000..4ffe37fe
--- /dev/null
+++ b/test/unit/blob-tool-reducer.test.js
@@ -0,0 +1,42 @@
+/* eslint-env jest */
+import brushReducer from '../../src/reducers/brush-tool';
+import eraserReducer from '../../src/reducers/eraser-tool';
+
+test('initialState', () => {
+    let defaultState;
+
+    expect(brushReducer(defaultState /* state */, {type: 'anything'} /* action */)).toBeDefined();
+    expect(brushReducer(defaultState /* state */, {type: 'anything'} /* action */).brushSize).toBeGreaterThan(0);
+
+    expect(eraserReducer(defaultState /* state */, {type: 'anything'} /* action */)).toBeTruthy();
+    expect(eraserReducer(defaultState /* state */, {type: 'anything'} /* action */).brushSize).toBeGreaterThan(0);
+});
+
+test('changeBrushSize', () => {
+    let defaultState;
+    const newBrushSize = 8078;
+
+    expect(brushReducer(defaultState /* state */, brushReducer.changeBrushSize(newBrushSize) /* action */))
+        .toEqual({brushSize: newBrushSize});
+    expect(brushReducer(1 /* state */, brushReducer.changeBrushSize(newBrushSize) /* action */))
+        .toEqual({brushSize: newBrushSize});
+
+    expect(eraserReducer(defaultState /* state */, eraserReducer.changeBrushSize(newBrushSize) /* action */))
+        .toEqual({brushSize: newBrushSize});
+    expect(eraserReducer(1 /* state */, eraserReducer.changeBrushSize(newBrushSize) /* action */))
+        .toEqual({brushSize: newBrushSize});
+});
+
+test('invalidChangeBrushSize', () => {
+    const origState = {brushSize: 1};
+
+    expect(brushReducer(origState /* state */, brushReducer.changeBrushSize('invalid argument') /* action */))
+        .toBe(origState);
+    expect(brushReducer(origState /* state */, brushReducer.changeBrushSize() /* action */))
+        .toBe(origState);
+
+    expect(eraserReducer(origState /* state */, eraserReducer.changeBrushSize('invalid argument') /* action */))
+        .toBe(origState);
+    expect(eraserReducer(origState /* state */, eraserReducer.changeBrushSize() /* action */))
+        .toBe(origState);
+});