diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
index b597d6adbd01c..e0bf05823f99a 100644
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -8887,7 +8887,7 @@ static bool combineCCMask(SDValue &CCReg, int &CCValid, int &CCMask,
       auto Result = Op0APVal & Op1APVal;
       bool AllOnes = Result == Op1APVal;
       bool AllZeros = Result == 0;
-      bool IsLeftMostBitSet = Result[Op1APVal.getActiveBits()] != 0;
+      bool IsLeftMostBitSet = Result[Op1APVal.getActiveBits() - 1] != 0;
       return AllZeros ? 0 : AllOnes ? 3 : IsLeftMostBitSet ? 2 : 1;
     };
     SDValue Op0 = CCNode->getOperand(0);
diff --git a/llvm/test/CodeGen/SystemZ/combine-ccmask-tm-full.ll b/llvm/test/CodeGen/SystemZ/combine-ccmask-tm-full.ll
new file mode 100644
index 0000000000000..85ab999d00bd7
--- /dev/null
+++ b/llvm/test/CodeGen/SystemZ/combine-ccmask-tm-full.ll
@@ -0,0 +1,41 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; Test that DAGCombiner doesn't crash when combining CCMask with TM instruction
+; where all bits in the mask are active (off-by-one error regression test).
+;
+; RUN: llc < %s -mtriple=s390x-linux-gnu -mcpu=z10 | FileCheck %s
+
+define void @test_tm_full_mask(ptr %ptr) {
+; CHECK-LABEL: test_tm_full_mask:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    lb %r0, 0(%r2)
+; CHECK-NEXT:    tmll %r0, 1
+; CHECK-NEXT:    lhi %r1, 1
+; CHECK-NEXT:    jne .LBB0_2
+; CHECK-NEXT:  # %bb.1:
+; CHECK-NEXT:    lr %r1, %r0
+; CHECK-NEXT:  .LBB0_2:
+; CHECK-NEXT:    tmll %r1, 1
+; CHECK-NEXT:    je .LBB0_4
+; CHECK-NEXT:  # %bb.3: # %if.then
+; CHECK-NEXT:    mvi 0(%r2), 1
+; CHECK-NEXT:  .LBB0_4: # %loop
+; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    j .LBB0_4
+  %1 = load i1, ptr %ptr, align 4
+  %2 = freeze i1 %1
+  %3 = select i1 %2, i32 0, i32 -1973148440
+  %4 = trunc i32 %3 to i16
+  %5 = or disjoint i16 %4, 1
+  %6 = icmp ult i16 %5, 3
+  %7 = and i32 %3, -1973157888
+  %8 = icmp eq i32 %7, 0
+  %9 = select i1 %6, i1 %8, i1 %2
+  br i1 %9, label %if.then, label %loop
+
+if.then:
+  store i1 true, ptr %ptr, align 4
+  br label %loop
+
+loop:
+  br label %loop
+}