diff --git a/llvm/test/tools/llvm-reduce/issue100880-mismatched-call-type.ll b/llvm/test/tools/llvm-reduce/issue100880-mismatched-call-type.ll
new file mode 100644
index 0000000000000..7854ed255ebe8
--- /dev/null
+++ b/llvm/test/tools/llvm-reduce/issue100880-mismatched-call-type.ll
@@ -0,0 +1,24 @@
+; RUN: llvm-reduce %s -o %t --abort-on-invalid-reduction --delta-passes=operands-to-args --test FileCheck --test-arg %s --test-arg --check-prefix=INTERESTING --test-arg --input-file
+; RUN: FileCheck %s --input-file %t --check-prefix=REDUCED
+
+@a = dso_local global i8 0, align 1
+@b = dso_local global i16 0, align 2
+
+; INTERESTING-LABEL: define void @c(
+; INTERESTING: sext
+; INTERESTING: icmp
+
+; REDUCED: define void @c(ptr %a, i8 %ld0, ptr %b, i16 %ld1, i32 %conv, i32 %conv1, i1 %cmp, i32 %conv2)
+; REDUCED: call void @c(i32 noundef signext %conv2)
+define void @c() {
+entry:
+  %ld0 = load i8, ptr @a, align 1
+  %conv = zext i8 %ld0 to i32
+  %ld1 = load i16, ptr @b, align 2
+  %conv1 = sext i16 %ld1 to i32
+  %cmp = icmp sge i32 %conv, %conv1
+  %conv2 = zext i1 %cmp to i32
+  call void @c(i32 noundef signext %conv2)
+  ret void
+}
+
diff --git a/llvm/test/tools/llvm-reduce/operands-to-args-wrong-call-type-issue69312.ll b/llvm/test/tools/llvm-reduce/operands-to-args-wrong-call-type-issue69312.ll
new file mode 100644
index 0000000000000..542737b08ce2b
--- /dev/null
+++ b/llvm/test/tools/llvm-reduce/operands-to-args-wrong-call-type-issue69312.ll
@@ -0,0 +1,44 @@
+; RUN: llvm-reduce %s -o %t --abort-on-invalid-reduction --delta-passes=operands-to-args --test FileCheck --test-arg %s --test-arg --match-full-lines --test-arg --check-prefix=INTERESTING --test-arg --input-file
+; RUN: FileCheck %s --input-file %t --check-prefixes=REDUCED,INTERESTING
+
+; REDUCED-LABEL: define void @func(i32 %k, i32 %extra.arg, ptr %Global, ptr %Local) {
+
+; Keep one reference to the original value.
+; INTERESTING: %[[LOCAL:Local[0-9]*]] = alloca i32, align 4
+
+; Everything else must use the function argument.
+; REDUCED: store i32 21, ptr %Global, align 4
+; REDUCED: store i32 0, ptr %Local, align 4
+; REDUCED: store i32 0, ptr %Global, align 4
+
+; Do not add any arguments for %Keep and @GlobalKeep.
+; INTERESTING: %[[KEEP:LocalKeep[0-9]*]] = add i32 %k, 21
+; INTERESTING: store i32 %[[KEEP]], ptr @GlobalKeep, align 4
+
+; Do not add any arguments if the call type was already mismatched
+
+; INTERESTING-LABEL: define void @mismatched_func_caller() {
+; REDUCED:             call void @func(i32 21)
+
+@Global = global i32 42
+@GlobalKeep = global i32 42
+
+define void @func(i32 %k, i32 %extra.arg) {
+entry:
+  %Local = alloca i32, align 4
+  store i32 21, ptr @Global, align 4
+  store i32 0, ptr %Local, align 4
+  store i32 0, ptr @Global, align 4
+  %LocalKeep = add i32 %k, 21
+  store i32 %LocalKeep, ptr @GlobalKeep, align 4
+  ret void
+}
+
+; This call has the wrong signature for the original underlying call,
+; so getCalledFunction does not return a reference to the function.
+define void @mismatched_func_caller() {
+entry:
+  call void @func(i32 21)
+  ret void
+}
+
diff --git a/llvm/tools/llvm-reduce/deltas/ReduceOperandsToArgs.cpp b/llvm/tools/llvm-reduce/deltas/ReduceOperandsToArgs.cpp
index aff0b849e8b83..66d4bd3048171 100644
--- a/llvm/tools/llvm-reduce/deltas/ReduceOperandsToArgs.cpp
+++ b/llvm/tools/llvm-reduce/deltas/ReduceOperandsToArgs.cpp
@@ -61,8 +61,20 @@ static void replaceFunctionCalls(Function *OldF, Function *NewF) {
   for (Use &U : OldF->uses()) {
     auto *CI = cast<CallBase>(U.getUser());
     assert(&U == &CI->getCalledOperandUse());
-    assert(CI->getCalledFunction() == OldF);
-    Callers.push_back(CI);
+
+    Function *CalledF = CI->getCalledFunction();
+    if (CalledF == OldF) {
+      Callers.push_back(CI);
+    } else {
+      // The call may have undefined behavior by calling a function with a
+      // mismatched signature. In this case, do not bother adjusting the
+      // callsites to pad with any new arguments.
+
+      // TODO: Better QoI to try to add new arguments to the end, and ignore
+      // existing mismatches.
+      assert(!CalledF && CI->getCalledOperand()->stripPointerCasts() == OldF &&
+             "only expected call and function signature mismatch");
+    }
   }
 
   // Call arguments for NewF.