diff --git a/src/app/commands/filters/cmd_despeckle.cpp b/src/app/commands/filters/cmd_despeckle.cpp
index 1bd51922e..982300e6f 100644
--- a/src/app/commands/filters/cmd_despeckle.cpp
+++ b/src/app/commands/filters/cmd_despeckle.cpp
@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2001-2015  David Capello
+// Copyright (C) 2001-2017  David Capello
 //
 // This program is distributed under the terms of
 // the End-User License Agreement for Aseprite.
@@ -60,8 +60,14 @@ public:
 private:
   void onSizeChange()
   {
-    m_filter.setSize(m_widthEntry->textInt(),
-                     m_heightEntry->textInt());
+    gfx::Size newSize(m_widthEntry->textInt(),
+                      m_heightEntry->textInt());
+
+    // Avoid negative numbers
+    newSize.w = MAX(1, newSize.w);
+    newSize.h = MAX(1, newSize.h);
+
+    m_filter.setSize(newSize.w, newSize.h);
     restartPreview();
   }
 
diff --git a/src/filters/median_filter.cpp b/src/filters/median_filter.cpp
index a6c9c46b8..cb2219606 100644
--- a/src/filters/median_filter.cpp
+++ b/src/filters/median_filter.cpp
@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2001-2015  David Capello
+// Copyright (C) 2001-2017  David Capello
 //
 // This program is distributed under the terms of
 // the End-User License Agreement for Aseprite.
@@ -90,8 +90,8 @@ namespace {
 
 MedianFilter::MedianFilter()
   : m_tiledMode(TiledMode::NONE)
-  , m_width(0)
-  , m_height(0)
+  , m_width(1)
+  , m_height(1)
   , m_ncolors(0)
   , m_channel(4)
 {
@@ -104,8 +104,11 @@ void MedianFilter::setTiledMode(TiledMode tiled)
 
 void MedianFilter::setSize(int width, int height)
 {
-  m_width = width;
-  m_height = height;
+  ASSERT(width >= 1);
+  ASSERT(height >= 1);
+
+  m_width = MAX(1, width);
+  m_height = MAX(1, height);
   m_ncolors = width*height;
 
   for (int c = 0; c < 4; ++c)