diff --git a/go/ssa/blockopt.go b/go/ssa/blockopt.go
index 64cfbeb8..e79260a2 100644
--- a/go/ssa/blockopt.go
+++ b/go/ssa/blockopt.go
@@ -117,6 +117,14 @@ func fuseBlocks(f *Function, a *BasicBlock) bool {
 	if len(b.Preds) != 1 {
 		return false
 	}
+
+	// Degenerate &&/|| ops may result in a straight-line CFG
+	// containing φ-nodes. (Ideally we'd replace such them with
+	// their sole operand but that requires Referrers, built later.)
+	if b.hasPhi() {
+		return false // not sound without further effort
+	}
+
 	// Eliminate jump at end of A, then copy all of B across.
 	a.Instrs = append(a.Instrs[:len(a.Instrs)-1], b.Instrs...)
 	for _, instr := range b.Instrs {
diff --git a/go/ssa/interp/testdata/coverage.go b/go/ssa/interp/testdata/coverage.go
index 5921f296..503e4e2f 100644
--- a/go/ssa/interp/testdata/coverage.go
+++ b/go/ssa/interp/testdata/coverage.go
@@ -636,3 +636,9 @@ func init() {
 		panic(got)
 	}
 }
+
+// Regression test for issue 7840 (covered by SSA sanity checker).
+func bug7840() bool {
+	// This creates a single-predecessor block with a φ-node.
+	return false && a == 0 && a == 0
+}