diff --git a/src/CBot/CBotInstr/CBotExpression.cpp b/src/CBot/CBotInstr/CBotExpression.cpp
index cb7f3dfc..c98deca5 100644
--- a/src/CBot/CBotInstr/CBotExpression.cpp
+++ b/src/CBot/CBotInstr/CBotExpression.cpp
@@ -71,6 +71,13 @@ CBotInstr* CBotExpression::Compile(CBotToken* &p, CBotCStack* pStack)
             return nullptr;
         }
 
+        if ( p->GetType() == ID_SEP )
+        {
+            pStack->SetError(CBotErrNoExpression, p);
+            delete inst;
+            return nullptr;
+        }
+
         inst->m_rightop = CBotExpression::Compile(p, pStack);
         if (inst->m_rightop == nullptr)
         {
@@ -118,13 +125,13 @@ CBotInstr* CBotExpression::Compile(CBotToken* &p, CBotCStack* pStack)
             break;
         case ID_ASSADD:
             if (type2.Eq(CBotTypBoolean) ||
-                type2.Eq(CBotTypPointer) ) type2 = -1;        // numbers and strings
+                type2.GetType() > CBotTypString ) type2.SetType(-1);    // numbers and strings
             break;
         case ID_ASSSUB:
         case ID_ASSMUL:
         case ID_ASSDIV:
         case ID_ASSMODULO:
-            if (type2.GetType() >= CBotTypBoolean) type2 = -1;    // numbers only
+            if (type2.GetType() >= CBotTypBoolean) type2.SetType(-1);    // numbers only
             break;
         }
 
@@ -179,6 +186,18 @@ bool CBotExpression::Execute(CBotStack* &pj)
     if ( pile2->GetState()==0)
     {
         if (m_rightop && !m_rightop->Execute(pile2)) return false;    // initial value // interrupted?
+        if (m_rightop)
+        {
+            CBotVar* var = pile1->GetVar();
+            CBotVar* value = pile2->GetVar();
+            if (var->GetType() == CBotTypString && value->GetType() != CBotTypString)
+            {
+                CBotVar* newVal = CBotVar::Create("", var->GetTypResult());
+                value->Update(pj->GetUserPtr());
+                newVal->SetValString(value->GetValString());
+                pile2->SetVar(newVal);
+            }
+        }
         pile2->IncState();
     }
 
diff --git a/src/CBot/CBotInstr/CBotInstrUtils.cpp b/src/CBot/CBotInstr/CBotInstrUtils.cpp
index 05de1a47..bbe80b27 100644
--- a/src/CBot/CBotInstr/CBotInstrUtils.cpp
+++ b/src/CBot/CBotInstr/CBotInstrUtils.cpp
@@ -97,9 +97,9 @@ bool TypeCompatible(CBotTypResult& type1, CBotTypResult& type2, int op)
     if (max == 99) return false;    // result is void?
 
     // special case for strin concatenation
-    if (op == ID_ADD && max >= CBotTypString) return true;
-    if (op == ID_ASSADD && max >= CBotTypString) return true;
-    if (op == ID_ASS && t1 == CBotTypString) return true;
+    if (op == ID_ADD && t1 == CBotTypString) return true;
+    if (op == ID_ASSADD && t2 == CBotTypString) return true;
+    if (op == ID_ASS && t2 == CBotTypString) return true;
 
     if (max >= CBotTypBoolean)
     {
diff --git a/src/CBot/CBotInstr/CBotInstrUtils.h b/src/CBot/CBotInstr/CBotInstrUtils.h
index a609988d..b5302883 100644
--- a/src/CBot/CBotInstr/CBotInstrUtils.h
+++ b/src/CBot/CBotInstr/CBotInstrUtils.h
@@ -39,7 +39,13 @@ CBotInstr* CompileParams(CBotToken* &p, CBotCStack* pStack, CBotVar** ppVars);
 
 /*!
  * \brief TypeCompatible Check if two results are consistent to make an
- * operation.
+ * operation. TypeCompatible is used in two ways:
+ * For non-assignment operations:  see CBotTwoOpExpr::Compile
+ * TypeCompatible( leftType, rightType, opType )
+
+ * For assignment or compound assignment operations (it's reversed):
+ * see CBotReturn::Compile & CBotExpression::Compile
+ * TypeCompatible( valueType, varType, opType ) 
  * \param type1
  * \param type2
  * \param op
diff --git a/src/CBot/CBotInstr/CBotLeftExprVar.cpp b/src/CBot/CBotInstr/CBotLeftExprVar.cpp
index 848d16a8..4394fc43 100644
--- a/src/CBot/CBotInstr/CBotLeftExprVar.cpp
+++ b/src/CBot/CBotInstr/CBotLeftExprVar.cpp
@@ -64,6 +64,12 @@ bool CBotLeftExprVar::Execute(CBotStack* &pj)
     CBotVar* var2 = pj->GetVar(); // Initial value on the stack
     if (var2 != nullptr)
     {
+        if (m_typevar.Eq(CBotTypString) && var2->GetType() != CBotTypString)
+        {
+            var2->Update(pj->GetUserPtr());
+            var1->SetValString(var2->GetValString());
+            return true;
+        }
         var1->SetVal(var2); // Set the value
     }
 
diff --git a/src/CBot/CBotInstr/CBotTwoOpExpr.cpp b/src/CBot/CBotInstr/CBotTwoOpExpr.cpp
index 23e11531..41321032 100644
--- a/src/CBot/CBotInstr/CBotTwoOpExpr.cpp
+++ b/src/CBot/CBotInstr/CBotTwoOpExpr.cpp
@@ -214,6 +214,13 @@ CBotInstr* CBotTwoOpExpr::Compile(CBotToken* &p, CBotCStack* pStack, int* pOpera
 
             type2 = pStk->GetTypResult();                       // what kind of results?
 
+            if ( type1.Eq(99) || type2.Eq(99) )                 // operand is void
+            {
+                pStack->SetError(CBotErrBadType2, &inst->m_token);
+                delete inst;
+                return nullptr;
+            }
+
             // what kind of result?
             int TypeRes = std::max( type1.GetType(CBotTypResult::GetTypeMode::NULL_AS_POINTER), type2.GetType(CBotTypResult::GetTypeMode::NULL_AS_POINTER) );
             if (typeOp == ID_ADD && type1.Eq(CBotTypString))
@@ -267,7 +274,7 @@ CBotInstr* CBotTwoOpExpr::Compile(CBotToken* &p, CBotCStack* pStack, int* pOpera
                         return pStack->Return(nullptr, pStk);
                     }
 
-                    if ( TypeRes != CBotTypString )
+                    if ( TypeRes != CBotTypString )                     // keep string conversion
                         TypeRes = std::max(type1.GetType(), type2.GetType());
                     inst = i;
                 }
@@ -370,7 +377,9 @@ bool CBotTwoOpExpr::Execute(CBotStack* &pStack)
     // what kind of result?
     int TypeRes = std::max(type1.GetType(), type2.GetType());
 
-    if ( GetTokenType() == ID_ADD && type1.Eq(CBotTypString) )
+    // see "any type convertible chain" in compile method
+    if ( GetTokenType() == ID_ADD &&
+        (type1.Eq(CBotTypString) || type2.Eq(CBotTypString)) )
     {
         TypeRes = CBotTypString;
     }
@@ -397,7 +406,8 @@ bool CBotTwoOpExpr::Execute(CBotStack* &pStack)
     CBotVar*    result = CBotVar::Create("", TypeRes);
 
     // creates a variable to perform the calculation in the appropriate type
-    TypeRes = std::max(type1.GetType(), type2.GetType());
+    if ( TypeRes != CBotTypString )                                     // keep string conversion
+        TypeRes = std::max(type1.GetType(), type2.GetType());
 
     if ( GetTokenType() == ID_ADD && type1.Eq(CBotTypString) )
     {