diff --git a/src/arch/alpha/isa/decoder.isa b/src/arch/alpha/isa/decoder.isa
index c5f47aaa2..584d64a3d 100644
--- a/src/arch/alpha/isa/decoder.isa
+++ b/src/arch/alpha/isa/decoder.isa
@@ -748,6 +748,7 @@ decode OPCODE default Unknown::unknown() {
                 int miscRegIndex = (ipr_index < NumInternalProcRegs) ?
                         IprToMiscRegIndex[ipr_index] : -1;
                 if(miscRegIndex < 0 || !IprIsReadable(miscRegIndex) ||
+                    miscRegIndex >= NumInternalProcRegs)
                         fault = new UnimplementedOpcodeFault;
                 else
                     Ra = xc->readMiscRegWithEffect(miscRegIndex, fault);
@@ -761,7 +762,8 @@ decode OPCODE default Unknown::unknown() {
             1: hw_mtpr({{
                 int miscRegIndex = (ipr_index < NumInternalProcRegs) ?
                         IprToMiscRegIndex[ipr_index] : -1;
-                if(miscRegIndex < 0 || !IprIsWritable(miscRegIndex))
+                if(miscRegIndex < 0 || !IprIsWritable(miscRegIndex)
+                    miscRegIndex >= NumInternalProcRegs)
                         fault = new UnimplementedOpcodeFault;
                 else
                     xc->setMiscRegWithEffect(miscRegIndex, Ra);