diff --git a/css/ui.css b/css/ui.css
index a8fe0a76..29a3a1d5 100644
--- a/css/ui.css
+++ b/css/ui.css
@@ -495,6 +495,32 @@ div.asset_file_header {
   border-radius: 8px;
   padding-left: 1em;
 }
+div.asset_block {
+  margin-top: 8px;
+}
+.asset_snip {
+  font-family: "Andale Mono", "Menlo", "Lucida Console", monospace;
+  font-weight: bold;
+  color: #c1c1b0;
+  background-color: #555;
+  border-radius: 8px;
+  padding-left: 1em;
+}
+.asset_lineno {
+  color: #99cc99;
+  display: inline-block;
+  min-width: 4rem;
+  text-align: right;
+  margin-right: 8px;
+}
+.asset_error_msg {
+  color: #ff6666;
+  background-color: #330000;
+  padding: 4px 8px;
+  margin: 2px 0;
+  border-left: 3px solid #ff0000;
+  font-family: monospace;
+}
 div.asset_grid {
   line-height:0;
   margin: 1em;
diff --git a/src/ide/pixeleditor.ts b/src/ide/pixeleditor.ts
index 01952193..72cbd9ff 100644
--- a/src/ide/pixeleditor.ts
+++ b/src/ide/pixeleditor.ts
@@ -192,6 +192,30 @@ export function convertWordsToImages(words: UintArray, fmt: PixelEditorImageForm
   return images;
 }
 
+export function validateAssetData(datastr: string, fmt): string | null {
+  var words = parseHexWords(convertToHexStatements(datastr));
+  if (fmt.w > 0 && fmt.h > 0) {
+    // same variables as convertWordsToImages
+    var count = fmt.count || 1;
+    var bpp = fmt.bpp || 1;
+    var nplanes = fmt.np || 1;
+    var bitsperword = fmt.bpw || 8;
+    var wordsperline = fmt.sl || Math.ceil(fmt.w * bpp / bitsperword);
+    var pofs = fmt.pofs || wordsperline * fmt.h * count;
+    var skip = fmt.skip || 0;
+    var wpimg = fmt.wpimg || wordsperline * fmt.h;
+    var required = wpimg * count + (nplanes > 1 ? (nplanes - 1) * pofs : 0) + skip;
+    if (words.length != required) {
+      return `Expected ${required} value(s), found ${words.length}`;
+    }
+  } else if (fmt.pal) {
+    if (words.length < 1) {
+      return `Palette requires at least 1 value, found ${words.length}`;
+    }
+  }
+  return null;
+}
+
 export function convertImagesToWords(images: Uint8Array[], fmt: PixelEditorImageFormat): number[] {
   if (fmt.destfmt) fmt = fmt.destfmt;
   var width = fmt.w;
diff --git a/src/ide/views/asseteditor.ts b/src/ide/views/asseteditor.ts
index 4978dfc5..38bf6107 100644
--- a/src/ide/views/asseteditor.ts
+++ b/src/ide/views/asseteditor.ts
@@ -6,6 +6,15 @@ import { hex, safeident, rgb2bgr } from "../../common/util";
 import * as pixed from "../pixeleditor";
 import Mousetrap = require('mousetrap');
 
+function getLineNumber(data: string, offset: number): number {
+  let line = 1;
+  for (let i = 0; i < offset && i < data.length; i++) {
+    if (data[i] === '\n') line++;
+  }
+  return line;
+}
+
+
 export class AssetEditorView implements ProjectView, pixed.EditorContext {
   maindiv: JQuery;
   cureditordiv: JQuery;
@@ -146,14 +155,21 @@ export class AssetEditorView implements ProjectView, pixed.EditorContext {
         end = data.indexOf(';', start); // C
       }
       //console.log(id, start, end, m[1], data.substring(start,end));
-      if (end > start) {
+      var line = getLineNumber(data, m.index);
+      var header = m[0];
+      if (end < 0) {
+        var closingDelim = platform_id.includes('verilog') ? '"end"' : m[0].startsWith(';;') ? '";;"' : '";"';
+        result.push({ fileid: id, header: header, line: line, error: `No closing ${closingDelim} found after asset header` });
+      } else if (end <= start) {
+        result.push({ fileid: id, header: header, line: line, error: `Empty data block after asset header` });
+      } else {
         try {
           var jsontxt = m[1].replace(/([A-Za-z]+):/g, '"$1":'); // fix lenient JSON
           var json = JSON.parse(jsontxt);
           // TODO: name?
-          result.push({ fileid: id, fmt: json, start: start, end: end });
+          result.push({ fileid: id, header: header, line: line, fmt: json, start: start, end: end });
         } catch (e) {
-          console.log(e);
+          result.push({ fileid: id, header: header, line: line, error: `Invalid asset format: ${e.message}` });
         }
       }
     }
@@ -304,7 +320,22 @@ export class AssetEditorView implements ProjectView, pixed.EditorContext {
     } else if (typeof data === 'string') {
       let textfrags = this.scanFileTextForAssets(fileid, data);
       for (let frag of textfrags) {
+        const block = $('<div class="asset_block"/>').appendTo(this.ensureFileDiv(fileid));
+        var snip = $('<div class="asset_snip"/>').appendTo(block);
+        $('<span class="asset_lineno"/>').text(frag.line).appendTo(snip);
+        snip.append(' ' + frag.header);
+        if (frag.error) {
+          $('<div class="asset_error_msg"/>').text(frag.error).appendTo(block);
+          continue;
+        }
         if (frag.fmt) {
+          // validate data block size before creating editors
+          const assetError = pixed.validateAssetData(data.substring(frag.start, frag.end), frag.fmt);
+          if (assetError) {
+            $('<div class="asset_error_msg"/>').text(assetError).appendTo(block);
+            continue;
+          }
+
           let label = fileid; // TODO: label
           let node: pixed.PixNode = new pixed.TextDataNode(projectWindows, fileid, label, frag.start, frag.end);
           let first = node;
@@ -317,19 +348,19 @@ export class AssetEditorView implements ProjectView, pixed.EditorContext {
             node = node.addRight(new pixed.NESNametableConverter(this));
             node = node.addRight(new pixed.Palettizer(this, { w: 8, h: 8, bpp: 4 }));
             const fmt = { w: 8 * (frag.fmt.w || 32), h: 8 * (frag.fmt.h || 30), count: 1 }; // TODO: can't do custom sizes
-            node = node.addRight(new pixed.MapEditor(this, newDiv(this.ensureFileDiv(fileid)), fmt));
+            node = node.addRight(new pixed.MapEditor(this, newDiv(block), fmt));
             this.registerAsset("nametable", first, 2);
             nassets++;
           }
           // is this a bitmap?
           else if (frag.fmt.w > 0 && frag.fmt.h > 0) {
-            this.addPixelEditor(this.ensureFileDiv(fileid), node, frag.fmt);
+            this.addPixelEditor(block, node, frag.fmt);
             this.registerAsset("charmap", first, 1);
             nassets++;
           }
           // is this a palette?
           else if (frag.fmt.pal) {
-            this.addPaletteEditor(this.ensureFileDiv(fileid), node, frag.fmt);
+            this.addPaletteEditor(block, node, frag.fmt);
             this.registerAsset("palette", first, 0);
             nassets++;
           }