diff --git a/README.md b/README.md
index 76c8b38..0cbc6f2 100644
--- a/README.md
+++ b/README.md
@@ -19,6 +19,10 @@ assert(satisfies('GPL-2.0', 'GPL-2.0+'))
 assert(satisfies('GPL-2.0-only', 'GPL-2.0+'))
 assert(satisfies('GPL-2.0', 'GPL-2.0-or-later'))
 
+assert(satisfies('GFDL-1.1-invariants-or-later', 'GFDL-1.2-invariants-only'))
+assert(satisfies('GFDL-1.1-invariants-or-later', 'GFDL-1.2-invariants-or-later'))
+assert(!satisfies('GFDL-1.1-invariants-only', 'GFDL-1.2-invariants-or-later'))
+
 assert(!satisfies(
   'GPL-2.0',
   'GPL-2.0+ WITH Bison-exception-2.2'
diff --git a/index.js b/index.js
index 0fd51df..d3023f4 100644
--- a/index.js
+++ b/index.js
@@ -61,11 +61,12 @@ var licensesAreCompatible = function (first, second) {
 function normalizeGPLIdentifiers (argument) {
   var license = argument.license
   if (license) {
+    var isGFDL = startsWith(license, 'GFDL-')
     if (endsWith(license, '-or-later')) {
-      argument.license = license.replace('-or-later', '')
+      if (!isGFDL) argument.license = license.replace('-or-later', '')
       argument.plus = true
     } else if (endsWith(license, '-only')) {
-      argument.license = license.replace('-only', '')
+      if (!isGFDL) argument.license = license.replace('-only', '')
       delete argument.plus
     }
   } else if (argument.left && argument.right) {
@@ -75,6 +76,10 @@ function normalizeGPLIdentifiers (argument) {
   return argument
 }
 
+function startsWith (string, substring) {
+  return string.indexOf(substring) === 0
+}
+
 function endsWith (string, substring) {
   return string.indexOf(substring) === string.length - substring.length
 }