MQL5 runtime error reference
MQL5 error 4003 officially means "Wrong parameter when calling a system function." A runtime call received a value outside that function contract, even though the source compiled. Identify the exact system function, compare every runtime value with its documented domain, and reject invalid handles, indexes, counts, enum values, or empty identifiers before the call. Capture GetLastError immediately around the failing call so an older error is not mistaken for the current failure.
Open the Coding Agent and choose MQL5 from the language menu.
bool CopyOneValue(const int handle,const int buffer,double &value)
{
if(handle==INVALID_HANDLE || buffer<0) return false;
double data[1];
ResetLastError();
int copied=CopyBuffer(handle,buffer,0,1,data);
if(copied!=1)
PrintFormat("CopyBuffer copied=%d error=%d",copied,GetLastError());
else
value=data[0];
return copied==1;
}A runtime call received a value outside that function contract, even though the source compiled. The numeric value is listed in the official runtime error table, which is separate from MetaEditor compilation errors and trade server return codes.
Reset the last-error state before the suspected call, save its direct return value, and read GetLastError immediately afterward. This produces a causal log instead of a stale code.
Preserve the exact parameters and terminal context that produced the failure. Check one precondition at a time before adding retries.
Identify the exact system function, compare every runtime value with its documented domain, and reject invalid handles, indexes, counts, enum values, or empty identifiers before the call.
After the repair, test both the expected path and a deliberately invalid input. The second test confirms that the diagnostic branch still exposes failures.
The wrapper rejects two invalid inputs and records the direct CopyBuffer result with the runtime code. Apply the same contract-first pattern to the actual failing system function.
A clean compile or a cleared error code does not prove strategy logic, broker compatibility, backtest quality, or live-trading safety. Reproduce the result in the target MetaTrader 5 terminal and account environment.
bool CopyOneValue(const int handle,const int buffer,double &value)
{
if(handle==INVALID_HANDLE || buffer<0) return false;
double data[1];
ResetLastError();
int copied=CopyBuffer(handle,buffer,0,1,data);
if(copied!=1)
PrintFormat("CopyBuffer copied=%d error=%d",copied,GetLastError());
else
value=data[0];
return copied==1;
}Pineify can generate or revise MQL5 source and use MQL5 compilation diagnostics during the coding workflow. Keep the exact error, source location, program type, and intended behavior in the request so the change remains reviewable.
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Call ResetLastError directly before the suspected function so the next code belongs to that operation.
Save the function return value, every input, and GetLastError. Confirm that the captured runtime code is 4003.
Log the called function name and every evaluated argument. Validate handles against INVALID_HANDLE. Check indexes and counts against current array or series sizes. Use the documented enum and data type for each parameter.
Compile the changed source, repeat the failing operation, and test the surrounding EA or indicator behavior in MetaTrader 5.
The official runtime error table defines 4003 as "Wrong parameter when calling a system function."
No in this context. It is a terminal runtime code. Compiler diagnostics and MqlTradeResult.retcode values use separate code tables.
The terminal stores the last runtime error. Resetting it before the suspected call makes the following GetLastError value easier to attribute to that call.
Last verified against the linked official references on . Compile and test the final source in the MetaTrader 5 build, broker, symbol, and account environment you plan to use.
Provide the exact diagnostic and intended behavior, inspect the generated change, compile it, and verify it in your MT5 test environment.
Open MQL5 Coding Agent