diff --git a/libs/core/screen.cpp b/libs/core/screen.cpp
index dff65e69..c7ab8d21 100644
--- a/libs/core/screen.cpp
+++ b/libs/core/screen.cpp
@@ -239,7 +239,7 @@ extern "C" void drawPanic(int code) {
 }
 
 bool isValidImage(Buffer buf) {
-    return buf->length >= 3 && buf->data[0] == 0xf0;
+    return buf != NULL && buf->length >= 3 && buf->data[0] == 0xf0;
 }
 
 /** Makes an image bound to a buffer. */
diff --git a/libs/core/screen.ts b/libs/core/screen.ts
index 3f9dad5a..70ca003f 100644
--- a/libs/core/screen.ts
+++ b/libs/core/screen.ts
@@ -112,10 +112,10 @@ namespace brick {
         let byteWidth = (currFont.charWidth + 7) >> 3
         let charSize = byteWidth * currFont.charHeight
         let imgBuf = output.createBuffer(2 + charSize)
-        let img = screen.imageOf(imgBuf)
         let double = (mode & Draw.Quad) ? 4 : (mode & Draw.Double) ? 2 : 1
         imgBuf[0] = 0xf0
         imgBuf[1] = currFont.charWidth
+        let img = screen.imageOf(imgBuf)
         while (cp < text.length) {
             let ch = text.charCodeAt(cp++)
             if (ch == 10) {