diff --git a/reference/antlr4 b/reference/antlr4
index a1b85fb2a..873001d30 160000
--- a/reference/antlr4
+++ b/reference/antlr4
@@ -1 +1 @@
-Subproject commit a1b85fb2ae7d6475ca83559f47978f2fefd13e4f
+Subproject commit 873001d30c2baa6148783f2ce3be1044c5275d7d
diff --git a/runtime/CSharp/Antlr4.Runtime/Dfa/DFAState.cs b/runtime/CSharp/Antlr4.Runtime/Dfa/DFAState.cs
index 457c28f2f..44381d993 100644
--- a/runtime/CSharp/Antlr4.Runtime/Dfa/DFAState.cs
+++ b/runtime/CSharp/Antlr4.Runtime/Dfa/DFAState.cs
@@ -197,11 +197,14 @@ namespace Antlr4.Runtime.Dfa
 
         public virtual DFAState GetTarget(int symbol)
         {
-            if (edges == null)
+            lock (this)
             {
-                return null;
+                if (edges == null)
+                {
+                    return null;
+                }
+                return edges[symbol];
             }
-            return edges[symbol];
         }
 
         public virtual void SetTarget(int symbol, DFAState target)
@@ -230,15 +233,18 @@ namespace Antlr4.Runtime.Dfa
 
         public virtual DFAState GetContextTarget(int invokingState)
         {
-            if (contextEdges == null)
+            lock (this)
             {
-                return null;
+                if (contextEdges == null)
+                {
+                    return null;
+                }
+                if (invokingState == PredictionContext.EmptyFullStateKey)
+                {
+                    invokingState = -1;
+                }
+                return contextEdges[invokingState];
             }
-            if (invokingState == PredictionContext.EmptyFullStateKey)
-            {
-                invokingState = -1;
-            }
-            return contextEdges[invokingState];
         }
 
         public virtual void SetContextTarget(int invokingState, DFAState target)
diff --git a/runtime/CSharp/Antlr4.Runtime/UnbufferedCharStream.cs b/runtime/CSharp/Antlr4.Runtime/UnbufferedCharStream.cs
index 3afe65650..ab75c892d 100644
--- a/runtime/CSharp/Antlr4.Runtime/UnbufferedCharStream.cs
+++ b/runtime/CSharp/Antlr4.Runtime/UnbufferedCharStream.cs
@@ -234,7 +234,7 @@ namespace Antlr4.Runtime
         {
             for (int i = 0; i < n; i++)
             {
-                if (this.n > 0 && data[this.n - 1] == IntStreamConstants.Eof)
+                if (this.n > 0 && data[this.n - 1] == (char)IntStreamConstants.Eof)
                 {
                     return i;
                 }
@@ -284,7 +284,7 @@ namespace Antlr4.Runtime
             {
                 throw new ArgumentOutOfRangeException();
             }
-            if (index > n)
+            if (index >= n)
             {
                 return IntStreamConstants.Eof;
             }
@@ -432,7 +432,7 @@ namespace Antlr4.Runtime
             if (interval.a < bufferStartIndex || interval.b >= bufferStartIndex + n)
             {
                 throw new NotSupportedException("interval " + interval + " outside buffer: " + bufferStartIndex
-                     + ".." + (bufferStartIndex + n));
+                     + ".." + (bufferStartIndex + n - 1));
             }
             // convert from absolute to local index
             int i = interval.a - bufferStartIndex;