diff --git a/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterBitFieldVMNode.java b/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterBitFieldVMNode.java
index 16525a24948..2059ff31fa4 100644
--- a/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterBitFieldVMNode.java
+++ b/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterBitFieldVMNode.java
@@ -655,6 +655,9 @@ public class RegisterBitFieldVMNode extends AbstractExpressionVMNode
     		 */
     		int startIdx = "GRP(".length(); //$NON-NLS-1$
             int endIdx = expression.indexOf(')', startIdx);
+            if ( startIdx == -1 || endIdx == -1 ) {
+            	return null;
+            }
             String remaining = expression.substring(endIdx+1);
             if ( ! remaining.startsWith(".REG(") ) { //$NON-NLS-1$
                 return null;
@@ -665,6 +668,9 @@ public class RegisterBitFieldVMNode extends AbstractExpressionVMNode
              */
             startIdx = ".REG(".length(); //$NON-NLS-1$
             endIdx = remaining.indexOf(')', startIdx);
+            if ( startIdx == -1 || endIdx == -1 ) {
+            	return null;
+            }
             remaining = remaining.substring(endIdx+1);
             
             /*
@@ -675,6 +681,9 @@ public class RegisterBitFieldVMNode extends AbstractExpressionVMNode
             }
             startIdx = ".BFLD(".length(); //$NON-NLS-1$
             endIdx = remaining.indexOf(')', startIdx);
+            if ( startIdx == -1 || endIdx == -1 ) {
+            	return null;
+            }
             String bitFieldName = remaining.substring(startIdx, endIdx);
             
             /*
diff --git a/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterGroupVMNode.java b/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterGroupVMNode.java
index de94c23924e..63ceeaa3ecd 100644
--- a/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterGroupVMNode.java
+++ b/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterGroupVMNode.java
@@ -359,6 +359,9 @@ public class RegisterGroupVMNode extends AbstractExpressionVMNode
     		 */
     		int startIdx = "GRP(".length(); //$NON-NLS-1$
             int endIdx = expression.indexOf(')', startIdx);
+            if ( startIdx == -1 || endIdx == -1 ) {
+            	return null;
+            }
             String groupName = expression.substring(startIdx, endIdx);
             return groupName.trim();
         }
diff --git a/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterVMNode.java b/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterVMNode.java
index 7469de513a0..9320c7f5464 100644
--- a/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterVMNode.java
+++ b/plugins/org.eclipse.dd.dsf.debug.ui/src/org/eclipse/dd/dsf/debug/internal/provisional/ui/viewmodel/register/RegisterVMNode.java
@@ -648,6 +648,9 @@ public class RegisterVMNode extends AbstractExpressionVMNode
     		 */
     		int startIdx = "GRP(".length(); //$NON-NLS-1$
             int endIdx = expression.indexOf(')', startIdx);
+            if ( startIdx == -1 || endIdx == -1 ) {
+            	return null;
+            }
             String remaining = expression.substring(endIdx+1);
             if ( ! remaining.startsWith(".REG(") ) { //$NON-NLS-1$
                 return null;
@@ -658,6 +661,9 @@ public class RegisterVMNode extends AbstractExpressionVMNode
              */
             startIdx = ".REG(".length(); //$NON-NLS-1$
             endIdx = remaining.indexOf(')', startIdx);
+            if ( startIdx == -1 || endIdx == -1 ) {
+            	return null;
+            }
             String regName = remaining.substring(startIdx,endIdx);
             return regName.trim();
         }