diff --git a/src/cmd/compile/internal/ssa/phiopt.go b/src/cmd/compile/internal/ssa/phiopt.go
index 8643fa584c3..db7b02275cf 100644
--- a/src/cmd/compile/internal/ssa/phiopt.go
+++ b/src/cmd/compile/internal/ssa/phiopt.go
@@ -154,7 +154,7 @@ func phioptint(v *Value, b0 *Block, reverse int) {
 	}
 	v.AddArg(a)
 
-	cvt := v.Block.NewValue1(v.Pos, OpCvtBoolToUint8, a.Type, a)
+	cvt := v.Block.NewValue1(v.Pos, OpCvtBoolToUint8, v.Block.Func.Config.Types.UInt8, a)
 	switch v.Type.Size() {
 	case 1:
 		v.reset(OpCopy)
diff --git a/src/cmd/compile/internal/ssa/prove.go b/src/cmd/compile/internal/ssa/prove.go
index 6c6be39d34b..ce7d689f935 100644
--- a/src/cmd/compile/internal/ssa/prove.go
+++ b/src/cmd/compile/internal/ssa/prove.go
@@ -1334,7 +1334,7 @@ func removeBranch(b *Block, branch branch) {
 // isNonNegative reports whether v is known to be greater or equal to zero.
 func isNonNegative(v *Value) bool {
 	if !v.Type.IsInteger() {
-		panic("isNonNegative bad type")
+		v.Fatalf("isNonNegative bad type: %v", v.Type)
 	}
 	// TODO: return true if !v.Type.IsSigned()
 	// SSA isn't type-safe enough to do that now (issue 37753).
diff --git a/test/fixedbugs/issue40746.go b/test/fixedbugs/issue40746.go
new file mode 100644
index 00000000000..235282fd90c
--- /dev/null
+++ b/test/fixedbugs/issue40746.go
@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f(x byte, b bool) byte {
+	var c byte
+	if b {
+		c = 1
+	}
+
+	if int8(c) < 0 {
+		x++
+	}
+	return x
+}