diff --git a/Changes b/Changes
index b2c6be97c..010e972a2 100644
--- a/Changes
+++ b/Changes
@@ -70,6 +70,7 @@ Verilator 5.041 devel
 * Fix timeprecision backward assignment (#6469). [Artur Bieniek, Antmicro Ltd.]
 * Fix splitting hierarchically referenced variables (#6475). [Ryszard Rozak, Antmicro Ltd.]
 * Fix Windows compilation of Verilator with spaces in the path (#6477). [Fabian Keßler-Schulz]
+* Fix PROTOTYPEMIS error on implicit logic (#6482). [Alex Solomatnikov]
 
 
 Verilator 5.040 2025-08-30
diff --git a/src/V3AstNodeDType.h b/src/V3AstNodeDType.h
index f7379bf61..806d39285 100644
--- a/src/V3AstNodeDType.h
+++ b/src/V3AstNodeDType.h
@@ -430,7 +430,7 @@ public:
     void dumpJson(std::ostream& str) const override;
     // width/widthMin/numeric compared elsewhere
     bool sameNode(const AstNode* samep) const override;
-    bool similarDTypeNode(const AstNodeDType* samep) const override { return sameNode(samep); }
+    bool similarDTypeNode(const AstNodeDType* samep) const override;
     string name() const override VL_MT_STABLE { return m.m_keyword.ascii(); }
     string prettyDTypeName(bool full) const override;
     const char* broken() const override {
diff --git a/src/V3AstNodes.cpp b/src/V3AstNodes.cpp
index e4d4a619c..15ea7eee0 100644
--- a/src/V3AstNodes.cpp
+++ b/src/V3AstNodes.cpp
@@ -258,7 +258,23 @@ int AstBasicDType::widthTotalBytes() const {
 
 bool AstBasicDType::sameNode(const AstNode* samep) const {
     const AstBasicDType* const sp = VN_DBG_AS(samep, BasicDType);
-    if (!(m == sp->m) || numeric() != sp->numeric()) return false;
+    if (!(m == sp->m)) return false;
+    if (numeric() != sp->numeric()) return false;
+    if (!rangep() && !sp->rangep()) return true;
+    return rangep() && rangep()->sameTree(sp->rangep());
+}
+bool AstBasicDType::similarDTypeNode(const AstNodeDType* samep) const {
+    if (sameNode(samep)) return true;
+    const AstBasicDType* const sp = VN_DBG_AS(samep, BasicDType);
+    if (!(m.m_keyword == sp->m.m_keyword
+          || (m.m_keyword == VBasicDTypeKwd::LOGIC_IMPLICIT
+              && sp->m.m_keyword == VBasicDTypeKwd::LOGIC)
+          || (m.m_keyword == VBasicDTypeKwd::LOGIC
+              && sp->m.m_keyword == VBasicDTypeKwd::LOGIC_IMPLICIT)))
+        return false;
+    if (!(m.m_nrange == sp->m.m_nrange)) return false;
+    // Squash so NOSIGN == UNSIGNED
+    if (numeric().isSigned() != sp->numeric().isSigned()) return false;
     if (!rangep() && !sp->rangep()) return true;
     return rangep() && rangep()->sameTree(sp->rangep());
 }
diff --git a/test_regress/t/t_class_extern.v b/test_regress/t/t_class_extern.v
index 7310da8ea..da50973d9 100644
--- a/test_regress/t/t_class_extern.v
+++ b/test_regress/t/t_class_extern.v
@@ -9,6 +9,7 @@ class Cls;
    extern function int ext_f_np;
    extern function int ext_f_p();
    extern function int ext_f_i(int in);
+   extern function ext_f_imp(in);
    extern static function int get_1();
    extern task ext_t_np;
    extern task ext_t_p();
@@ -37,6 +38,10 @@ function int Cls::ext_f_i(int in);
    return in+1;
 endfunction
 
+function Cls::ext_f_imp(in);
+   return ~in;
+endfunction
+
 function int Cls::get_1();
    return 1;
 endfunction
@@ -88,6 +93,7 @@ module t;
       if (c.ext_f_np() != 1) $stop;
       if (c.ext_f_p() != 2) $stop;
       if (c.ext_f_i(10) != 11) $stop;
+      if (c.ext_f_imp(1'b1) != 1'b0) $stop;
       if (Cls::get_1() != 1) $stop;
       subc.ext_t_i(20);
       if (subc.ext_f_np() != 10) $stop;