diff --git a/README.md b/README.md
index 598f6bb5a..facc35e40 100644
--- a/README.md
+++ b/README.md
@@ -313,12 +313,13 @@ Configuration of build plans are organized under `[buildPlans.<plan name>]` sect
 * `family`: String, defines the family name of your custom variant.
 * `spacing`: Optional, String, denotes the spacing of the custom variant. Valid values include:
   - `term`: Make the symbols' width suitable for terminal emulators. Arrows and geometric symbols ill become narrower.
-  - `fontconfig-mono`: Apply `term` spacing changes and further:
+  - `fontconfig-mono`: Apply `term` spacing changes and further apply changes to be compatible with FontConfig's Mono spacing, which recognizes a font as monospace if and only if its every non-combining characters having the same width. The changes include:
     - Completely remove wide glyphs. All non-combining glyphs will be exactly the same width.
+      - As a consequence, the following characters will be **removed**:
+        - `U+27F5` LONG LEFTWARDS ARROW
+        - `U+27F6` LONG RIGHTWARDS ARROW
     - Remove `NWID` and `WWID` OpenType feature.
-    
-    This spacing is recommended for Linux users who customize for their terminal fonts: certain applications, including FontConfig, recognizes a font as monospace if and only if its every non-combining glyphs having the same width.
-  - `fixed`: Apply `fontconfig-mono` changes and remove ligations.
+  - `fixed`: Apply `fontconfig-mono` changes and further remove ligations.
 * `serifs`: Optional, String, configures style of serifs.
   - When set to `slab`, the font will be converted into slab-serif.
   - Otherwise the font will be sans-serif.
diff --git a/font-src/gen/finalize/index.js b/font-src/gen/finalize/index.js
index ee98b1f20..64eec0971 100644
--- a/font-src/gen/finalize/index.js
+++ b/font-src/gen/finalize/index.js
@@ -4,19 +4,19 @@ const finalizeGlyphs = require("./glyphs");
 const gcFont = require("./gc");
 
 module.exports = function finalizeFont(para, glyphStore, excludedCodePoints, restFont) {
-	glyphStore = forceMonospaceIfNeeded(para, glyphStore);
 	glyphStore = gcFont(glyphStore, excludedCodePoints, restFont, {});
 	glyphStore = finalizeGlyphs(para, glyphStore);
+	validateMonospace(para, glyphStore);
 	return glyphStore;
 };
 
-function forceMonospaceIfNeeded(para, glyphStore) {
-	const unitWidth = Math.round(para.width);
-	if (!para.forceMonospace || para.spacing > 0) return glyphStore;
-	return glyphStore.filterByGlyph({
-		has: g => {
-			const gw = Math.round(g.advanceWidth || 0);
-			return gw === 0 || gw === unitWidth;
-		}
-	});
+function validateMonospace(para, glyphStore) {
+	if (!para.forceMonospace) return;
+	let awSet = new Set();
+	for (const g of glyphStore.glyphs()) {
+		awSet.add(Math.round(g.advanceWidth || 0));
+	}
+	if (awSet.size > 2) {
+		throw new Error("Unreachable! Fixed variant has wide characters");
+	}
 }
diff --git a/font-src/glyphs/symbol/arrow.ptl b/font-src/glyphs/symbol/arrow.ptl
index 9f135a241..50ef46ecd 100644
--- a/font-src/glyphs/symbol/arrow.ptl
+++ b/font-src/glyphs/symbol/arrow.ptl
@@ -597,6 +597,7 @@ glyph-block Symbol-Arrow : for-width-kinds WideWidth1
 
 	if (MosaicWidthScalar == 1) : begin
 		glyph-block-export ArrowShape
-	if (MosaicNameSuffix === '.WWID') : begin
+
+	if (!para.forceMonospace && MosaicNameSuffix === '.WWID') : begin
 		alias 'longArrowLeft'  0x27F5 'arrowleft.WWID'
 		alias 'longArrowRight' 0x27F6 'arrowright.WWID'