diff --git a/.gitignore b/.gitignore
index d532e7bf..af1a159a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,4 +3,4 @@
 coverage/
 .nyc_output/
 .idea
-launch.json
\ No newline at end of file
+.vscode/launch.json
\ No newline at end of file
diff --git a/.vscode/launch.json b/.vscode/launch.json
index 47e3057c..591e718a 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -10,9 +10,9 @@
       "name": "Launch Program",
       "program": "${workspaceFolder}/bin/semver",
       "args": [
-        "-i", "preminor", 
+        "-i", "prerelease", 
         "--preid", "dev",
-        "-n", "1",
+        "-n", "0",
         "1.2.0"
     
       ]
diff --git a/semver.js b/semver.js
index 490c45f9..4b004ec3 100644
--- a/semver.js
+++ b/semver.js
@@ -461,8 +461,9 @@ SemVer.prototype.inc = function(release, identifier, identifierIndex) {
     // This probably shouldn't be used publicly.
     // 1.0.0 "pre" would become 1.0.0-0 which is the wrong direction.
     case 'pre':
-      if (identifierIndex) {
-          this.prerelease = [identifierIndex];
+      if (identifierIndex && validateIdentifierIndex(identifierIndex)) {
+        index = identifierIndex;
+        this.prerelease = [identifierIndex];
       }
       if (this.prerelease.length === 0)
         this.prerelease = [0];
@@ -481,10 +482,10 @@ SemVer.prototype.inc = function(release, identifier, identifierIndex) {
         //if user specified identifierIndex, use their value.  
         // Zero-based or one-based.  Default is zero.
         var index = 0;
-        if (typeof identifierIndex === 'number' &&
-          isNaN(identifierIndex) == false) {
-            index = identifierIndex;
-          }
+        // validation for identifierIndex done above
+        if (identifierIndex) {
+          index = identifierIndex;
+        }
         // 1.2.0-beta.1 bumps to 1.2.0-beta.2,
         // 1.2.0-beta.fooblz or 1.2.0-beta bumps to 1.2.0-beta.0
         if (this.prerelease[0] === identifier) {
@@ -510,15 +511,28 @@ function inc(version, release, loose, identifier, identifierIndex) {
     identifier = loose;
     loose = undefined;
   }
+  if (identifierIndex) identifierIndex = parseInt(identifierIndex);
 
   try {
-    identifierIndex = parseInt(identifierIndex);
     return new SemVer(version, loose).inc(release, identifier, identifierIndex).version;
   } catch (er) {
     return null;
   }
 }
 
+function validateIdentifierIndex (identifierIndex) {
+  if (typeof identifierIndex === 'number' &&
+    isNaN(identifierIndex) == false) {
+    if (identifierIndex === 1 || identifierIndex === 0) {
+      return true;
+    }  else {      
+      throw new TypeError('Invalid identifier number base: ' + identifierIndex);
+    }
+  } else {
+    throw new TypeError('Invalid identifier number base: ' + identifierIndex);
+  }
+}
+
 exports.diff = diff;
 function diff(version1, version2) {
   if (eq(version1, version2)) {