diff --git a/llvm/test/Reduce/do-not-remove-terminator.ll b/llvm/test/Reduce/do-not-remove-terminator.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Reduce/do-not-remove-terminator.ll
@@ -0,0 +1,19 @@
+; RUN: llvm-reduce --test FileCheck --test-arg --check-prefixes=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t
+; RUN: cat %t | FileCheck --check-prefixes=CHECK-FINAL %s
+
+; Make sure we do not remove the terminator of the entry block. The interesting
+; check only requires the result to define the function @test.
+
+; Test case for PR43798.
+
+; CHECK-INTERESTINGNESS: define i32 @test
+
+; CHECK-FINAL:      define i32 @test
+; CHECK-FINAL-NEXT: entry:
+; CHECK-FINAL-NEXT:   ret i32
+
+define i32 @test(i32 %x) {
+entry:
+  %add = add i32 %x, %x
+  ret i32 %add
+}
diff --git a/llvm/test/Reduce/remove-funcs.ll b/llvm/test/Reduce/remove-funcs.ll
--- a/llvm/test/Reduce/remove-funcs.ll
+++ b/llvm/test/Reduce/remove-funcs.ll
@@ -20,6 +20,7 @@
 
 ; CHECK-FINAL-NEXT: entry:
 ; CHECK-FINAL-NEXT:   %call2 = call i32 @interesting()
+; CHECK-FINAL-NEXT:   ret i32 5
 ; CHECK-FINAL-NEXT: }
 
 define i32 @uninteresting2() {
diff --git a/llvm/test/Reduce/remove-instructions.ll b/llvm/test/Reduce/remove-instructions.ll
--- a/llvm/test/Reduce/remove-instructions.ll
+++ b/llvm/test/Reduce/remove-instructions.ll
@@ -4,8 +4,12 @@
 ; RUN: cat %t | FileCheck -implicit-check-not=uninteresting %s
 ; REQUIRES: plugins
 
-; We're testing all direct uses of %interesting are conserved
+; We're testing all direct uses of %interesting are conserved. The terminator
+; (ret) must also be preserved.
+
 ; CHECK-COUNT-5: %interesting
+; CHECK: ret
+
 define i32 @main() #0 {
 entry:
   %uninteresting1 = alloca i32, align 4
@@ -18,6 +22,5 @@
   store i32 %uninteresting3, i32* %interesting, align 4
   %1 = load i32, i32* %interesting, align 4
   store i32 %1, i32* %uninteresting2, align 4
-  ; CHECK-NOT: ret
   ret i32 0
 }
diff --git a/llvm/tools/llvm-reduce/deltas/ReduceInstructions.cpp b/llvm/tools/llvm-reduce/deltas/ReduceInstructions.cpp
--- a/llvm/tools/llvm-reduce/deltas/ReduceInstructions.cpp
+++ b/llvm/tools/llvm-reduce/deltas/ReduceInstructions.cpp
@@ -24,10 +24,14 @@
   std::set<Instruction *> InstToKeep;
 
   for (auto &F : *Program)
-    for (auto &BB : F)
-      for (auto &Inst : BB)
+    for (auto &BB : F) {
+      // Removing the terminator would make the block invalid. Only iterate over
+      // instructions before the terminator.
+      InstToKeep.insert(BB.getTerminator());
+      for (auto &Inst : make_range(BB.begin(), std::prev(BB.end())))
         if (O.shouldKeep())
           InstToKeep.insert(&Inst);
+    }
 
   std::vector<Instruction *> InstToDelete;
   for (auto &F : *Program)
@@ -49,7 +53,8 @@
   int InstCount = 0;
   for (auto &F : *Program)
     for (auto &BB : F)
-        InstCount += BB.getInstList().size();
+      // Well-formed blocks have terminators, which we cannot remove.
+      InstCount += BB.getInstList().size() - 1;
   outs() << "Number of instructions: " << InstCount << "\n";
 
   return InstCount;