diff --git a/src/script_opt/FuncInfo.cc b/src/script_opt/FuncInfo.cc
index e27dc8282c..6b89e4c3a6 100644
--- a/src/script_opt/FuncInfo.cc
+++ b/src/script_opt/FuncInfo.cc
@@ -550,22 +550,23 @@ static std::unordered_map<std::string, unsigned int> func_attrs = {
 // Some of these have side effects that could be checked for in a specific
 // context, but the gains from doing so likely aren't worth the complexity.
 
-bool is_special_script_func(std::string func_name) {
+bool is_special_script_func(const std::string& func_name) {
     auto f_attr = func_attrs.find(func_name);
     return f_attr != func_attrs.end() && (f_attr->second & ATTR_SPECIAL_SCRIPT_FUNC) != 0;
 }
 
-bool is_idempotent(std::string func_name) {
+bool is_idempotent(const std::string& func_name) {
     auto f_attr = func_attrs.find(func_name);
     return f_attr != func_attrs.end() && (f_attr->second & ATTR_IDEMPOTENT) != 0;
 }
 
-bool has_no_script_side_effects(std::string func_name) {
+bool has_script_side_effects(const std::string& func_name) {
     auto f_attr = func_attrs.find(func_name);
     if ( f_attr == func_attrs.end() )
-        return false;
+        // We don't know about it, so be conservative.
+        return true;
 
-    return (f_attr->second & (ATTR_NO_SCRIPT_SIDE_EFFECTS | ATTR_NO_ZEEK_SIDE_EFFECTS | ATTR_IDEMPOTENT)) != 0;
+    return (f_attr->second & (ATTR_NO_SCRIPT_SIDE_EFFECTS | ATTR_NO_ZEEK_SIDE_EFFECTS | ATTR_IDEMPOTENT)) == 0;
 }
 
 } // namespace zeek::detail
diff --git a/src/script_opt/FuncInfo.h b/src/script_opt/FuncInfo.h
index f4bf333cc0..53095d227d 100644
--- a/src/script_opt/FuncInfo.h
+++ b/src/script_opt/FuncInfo.h
@@ -10,15 +10,15 @@ namespace zeek::detail {
 
 // A "special script function" is one that the event engine explicitly
 // knows about.
-extern bool is_special_script_func(std::string func_name);
+extern bool is_special_script_func(const std::string& func_name);
 
 // An idempotent function returns the same value when called with the
 // same arguments (and has no meaningful side effects in terms of script-level
 // or Zeek-internal state).
-extern bool is_idempotent(std::string func_name);
+extern bool is_idempotent(const std::string& func_name);
 
-// Whether the given function (currently, just BiFs) has no Zeek-script-level
+// Whether the given function (currently, just BiFs) has Zeek-script-level
 // side effects.
-extern bool has_no_script_side_effects(std::string func_name);
+extern bool has_script_side_effects(const std::string& func_name);
 
 } // namespace zeek::detail
diff --git a/src/script_opt/ProfileFunc.cc b/src/script_opt/ProfileFunc.cc
index 0457482703..df83fe1dd8 100644
--- a/src/script_opt/ProfileFunc.cc
+++ b/src/script_opt/ProfileFunc.cc
@@ -635,7 +635,7 @@ bool ProfileFuncs::GetCallSideEffects(const NameExpr* n, IDSet& non_local_ids, T
 
     auto func = fv->AsFunc();
     if ( func->GetKind() == Func::BUILTIN_FUNC ) {
-        if ( ! has_no_script_side_effects(func->Name()) )
+        if ( has_script_side_effects(func->Name()) )
             is_unknown = true;
         return true;
     }
@@ -1168,7 +1168,7 @@ bool ProfileFuncs::DefinitelyHasNoSideEffects(const ExprPtr& e) const {
         return false;
 
     for ( auto& b : pf->BiFGlobals() )
-        if ( ! has_no_script_side_effects(b->Name()) )
+        if ( has_script_side_effects(b->Name()) )
             return false;
 
     return true;
@@ -1259,7 +1259,7 @@ bool ProfileFuncs::AssessSideEffects(const ProfileFunc* pf, IDSet& non_local_ids
     }
 
     for ( auto& b : pf->BiFGlobals() )
-        if ( ! has_no_script_side_effects(b->Name()) ) {
+        if ( has_script_side_effects(b->Name()) ) {
             is_unknown = true;
             return true;
         }