diff --git a/CHANGELOG.md b/CHANGELOG.md
index fa5064186..2cd02aeb5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -37,6 +37,9 @@ Breaking API changes:
      * `textDocument/documentHighlight` changed to `DocumentHighlightParams`
      * `textDocument/prepareRename` changed to `PrepareRenameParams`
 
+  * The LSP's Protocol `Diagnostic` field `code` was previouslyt incorrectly declared as `String`,
+    it is now correctly declared as `Either<String, Number>`
+
 ### v0.8.1 (Sep. 2019)
 
 Fixed issues: https://github.com/eclipse/lsp4j/milestone/16?closed=1
diff --git a/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/Protocol.xtend b/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/Protocol.xtend
index bd417e3bf..6d6600195 100644
--- a/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/Protocol.xtend
+++ b/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/Protocol.xtend
@@ -1678,7 +1678,7 @@ class Diagnostic {
 	/**
 	 * The diagnostic's code. Can be omitted.
 	 */
-	String code
+	Either<String, Number> code
 
 	/**
 	 * A human-readable string describing the source of this diagnostic, e.g. 'typescript' or 'super lint'.
diff --git a/org.eclipse.lsp4j/src/main/xtend-gen/org/eclipse/lsp4j/Diagnostic.java b/org.eclipse.lsp4j/src/main/xtend-gen/org/eclipse/lsp4j/Diagnostic.java
index 52f9025fc..d3abb4aa6 100644
--- a/org.eclipse.lsp4j/src/main/xtend-gen/org/eclipse/lsp4j/Diagnostic.java
+++ b/org.eclipse.lsp4j/src/main/xtend-gen/org/eclipse/lsp4j/Diagnostic.java
@@ -16,6 +16,7 @@
 import org.eclipse.lsp4j.DiagnosticSeverity;
 import org.eclipse.lsp4j.DiagnosticTag;
 import org.eclipse.lsp4j.Range;
+import org.eclipse.lsp4j.jsonrpc.messages.Either;
 import org.eclipse.lsp4j.jsonrpc.validation.NonNull;
 import org.eclipse.lsp4j.util.Preconditions;
 import org.eclipse.xtext.xbase.lib.Pure;
@@ -41,7 +42,7 @@ public class Diagnostic {
   /**
    * The diagnostic's code. Can be omitted.
    */
-  private String code;
+  private Either<String, Number> code;
   
   /**
    * A human-readable string describing the source of this diagnostic, e.g. 'typescript' or 'super lint'.
@@ -85,7 +86,7 @@ public Diagnostic(@NonNull final Range range, @NonNull final String message, fin
   
   public Diagnostic(@NonNull final Range range, @NonNull final String message, final DiagnosticSeverity severity, final String source, final String code) {
     this(range, message, severity, source);
-    this.code = code;
+    this.setCode(code);
   }
   
   /**
@@ -125,17 +126,33 @@ public void setSeverity(final DiagnosticSeverity severity) {
    * The diagnostic's code. Can be omitted.
    */
   @Pure
-  public String getCode() {
+  public Either<String, Number> getCode() {
     return this.code;
   }
   
   /**
    * The diagnostic's code. Can be omitted.
    */
-  public void setCode(final String code) {
+  public void setCode(final Either<String, Number> code) {
     this.code = code;
   }
   
+  public void setCode(final String code) {
+    if (code == null) {
+      this.code = null;
+      return;
+    }
+    this.code = Either.forLeft(code);
+  }
+  
+  public void setCode(final Number code) {
+    if (code == null) {
+      this.code = null;
+      return;
+    }
+    this.code = Either.forRight(code);
+  }
+  
   /**
    * A human-readable string describing the source of this diagnostic, e.g. 'typescript' or 'super lint'.
    */