diff --git a/src/Stmt.h b/src/Stmt.h
index b8e52860b8..d79a8631fb 100644
--- a/src/Stmt.h
+++ b/src/Stmt.h
@@ -471,7 +471,7 @@ protected:
     std::vector<StmtPtr> stmts;
 
     // Optimization-related:
-    bool ReduceStmt(int& s_i, std::vector<StmtPtr>& f_stmts, Reducer* c);
+    bool ReduceStmt(unsigned int& s_i, std::vector<StmtPtr>& f_stmts, Reducer* c);
 
     void ResetStmts(std::vector<StmtPtr> new_stmts) { stmts = std::move(new_stmts); }
 };
diff --git a/src/script_opt/CPP/Types.cc b/src/script_opt/CPP/Types.cc
index 6b72851f83..f97987a9e9 100644
--- a/src/script_opt/CPP/Types.cc
+++ b/src/script_opt/CPP/Types.cc
@@ -114,7 +114,7 @@ const char* CPPCompile::TypeName(const TypePtr& t) {
         case TYPE_BOOL: return "bool";
         case TYPE_COUNT: return "zeek_uint_t";
         case TYPE_DOUBLE: return "double";
-        case TYPE_ENUM: return "int";
+        case TYPE_ENUM: return "zeek_int_t";
         case TYPE_INT: return "zeek_int_t";
         case TYPE_INTERVAL: return "double";
         case TYPE_PORT: return "zeek_uint_t";
diff --git a/src/script_opt/Stmt.cc b/src/script_opt/Stmt.cc
index 7cb59cd6c6..444bda789f 100644
--- a/src/script_opt/Stmt.cc
+++ b/src/script_opt/Stmt.cc
@@ -632,7 +632,7 @@ void StmtList::Inline(Inliner* inl) {
 bool StmtList::IsReduced(Reducer* c) const {
     auto n = stmts.size();
 
-    for ( auto i = 0; i < n; ++i ) {
+    for ( auto i = 0U; i < n; ++i ) {
         auto& s_i = stmts[i];
         if ( ! s_i->IsReduced(c) )
             return false;
@@ -650,7 +650,7 @@ StmtPtr StmtList::DoReduce(Reducer* c) {
 
     auto n = stmts.size();
 
-    for ( auto i = 0; i < n; ++i ) {
+    for ( auto i = 0U; i < n; ++i ) {
         if ( ReduceStmt(i, f_stmts, c) )
             did_change = true;
 
@@ -677,7 +677,7 @@ StmtPtr StmtList::DoReduce(Reducer* c) {
     return ThisPtr();
 }
 
-bool StmtList::ReduceStmt(int& s_i, std::vector<StmtPtr>& f_stmts, Reducer* c) {
+bool StmtList::ReduceStmt(unsigned int& s_i, std::vector<StmtPtr>& f_stmts, Reducer* c) {
     bool did_change = false;
     auto& stmt_i = stmts[s_i];
     auto old_stmt = stmt_i.get();
diff --git a/src/script_opt/UseDefs.cc b/src/script_opt/UseDefs.cc
index b491886021..855d7145f3 100644
--- a/src/script_opt/UseDefs.cc
+++ b/src/script_opt/UseDefs.cc
@@ -182,7 +182,7 @@ UDs UseDefs::PropagateUDs(const Stmt* s, UDs succ_UDs, const Stmt* succ_stmt, bo
 
                 const Stmt* succ;
 
-                if ( i == stmts.size() - 1 ) { // Very last statement.
+                if ( i == int(stmts.size()) - 1 ) { // Very last statement.
                     succ = succ_stmt;
                     if ( successor2.find(s) != successor2.end() )
                         successor2[s_i] = successor2[s];
diff --git a/src/script_opt/ZAM/Ops.in b/src/script_opt/ZAM/Ops.in
index 0a774fa4f4..e407676ba4 100644
--- a/src/script_opt/ZAM/Ops.in
+++ b/src/script_opt/ZAM/Ops.in
@@ -981,7 +981,7 @@ macro EvalIndexVec(index)
 	zeek_int_t ind = index;
 	if ( ind < 0 )
 		ind += vv.size();
-	if ( ind < 0 || ind >= vv.size() )
+	if ( ind < 0 || ind >= int(vv.size()) )
 		ZAM_run_time_error(z.loc, "no such index");
 	AssignV1(CopyVal(*vec[ind]))
 
@@ -998,7 +998,7 @@ macro EvalIndexAnyVec(index)
 	zeek_int_t ind = index;
 	if ( ind < 0 )
 		ind += vv->Size();
-	if ( ind < 0 || ind >= vv->Size() )
+	if ( ind < 0 || ind >= int(vv->Size()) )
 		ZAM_run_time_error(z.loc, "no such index");
 	AssignV1(ZVal(vv->ValAt(ind).release()))
 
diff --git a/src/script_opt/ZAM/ZBody.cc b/src/script_opt/ZAM/ZBody.cc
index 0643801d09..42db4a58f4 100644
--- a/src/script_opt/ZAM/ZBody.cc
+++ b/src/script_opt/ZAM/ZBody.cc
@@ -233,7 +233,7 @@ ValPtr ZBody::Exec(Frame* f, StmtFlowType& flow) {
 }
 
 ValPtr ZBody::DoExec(Frame* f, StmtFlowType& flow) {
-    int pc = 0;
+    unsigned int pc = 0;
 
     // Return value, or nil if none.
     const ZVal* ret_u = nullptr;