The following table includes all the syntax error messages that the APL compiler (Aplc.exe) can generate. Where necessary, the table provides a description of the error message.
| Error message | Description |
| Division by 0. | |
| Duplicate label "%s". | This line is the second (or more) line that defines a label %s:, where %s represents the actual label name. |
| Duplicate variable name "%s". | This variable has already been declared; %s represents the actual variable name. |
| ELSE with no IF. | |
| ELSEIF found after ELSE. | |
| ELSEIF without matching IF. | |
| ENDIF with no IF. | |
| ENDWHILE with no WHILE. | |
| Extra characters on line. | The compiler found what appeared to be a valid command, but there were additional characters remaining after compilation was complete. This can be a confusing error because an error in the first part of the line can appear as a different command to the compiler. |
| FOR at line %d has no NEXT %s. | %d represents the line number where the For command was found and %s represents the control variable in the For command. |
| GOSUB expected. | An OnCrash, OnStall, or OnKey command is missing the GoSub keyword. |
| IF at line %d has no ENDIF. | %d represents the line number of the If command. |
| Illegal character "%c". | %c represents the illegal character. |
| Illegal character. | The line contains a non-ASCII character. |
| Invalid coverage for this cloud type. | The coverage keyword is not valid for the type of clouds specified in this Clouds command. See the Clouds Command topic for details. |
| Invalid number. | The line contains a string when the compiler expected a number, and it could not be evaluated. For example, more than one decimal point in a number would cause this error. |
| KEYPRESS expected. | The keypress parameter is missing from the Wait command. This error occurs when the compiler finds a time value and a comma, and then either the end of the line or something other than the keypress parameter. This error is also returned if the parameter is misspelled. |
| Key specification expected. | The KV( ) function didn't understand its argument. See the Key Value Specification topic for details. |
| Keyword expected. | The compiler found something other than a word when a keyword was expected. This error can be caused by a punctuation error. |
| Label name expected. | The compiler found something other than a word when it expected a label name. This error can be caused by a punctuation error or an error in the label name. Label names must start with a letter and can contain only letters, digits, and underscores. |
| Line too long. | Lines can't be longer than 255 characters (not counting the carriage return/linefeed characters). |
| Missing ")". | This line has more opening parentheses than closing parentheses. |
| Missing label "%s" referenced at line %d. | A GoTo, GoSub, OnCrash, OnStall, or OnKey command on line %d (where %d represents the line number) referenced a label named %s (where %s represents the actual name of the label), but no label by that name was found. Check for spelling errors in both the reference to the label and the definition. |
| NEXT variable does not match FOR. | The variable specified in the For command must match the variable specified in the Next command. This error can be caused by a missing Next command or a spelling error. |
| NEXT without matching FOR. | |
| Only one TITLE per adventure. | |
| Operand expected. | The compiler couldn't find an expected operand (a variable, a number, or an expression in parentheses). This error can be caused by a punctuation error, two operators in a row, or a line ending in an operator. |
| Quoted string expected. | This command expects a string in quotation marks. This error can be caused by leaving out the quotation marks. |
| String expected. | A Declare <variable> = statement was followed by something other than a quoted string. |
| String in KV must be1 character long. | If the argument for KV( ) is a quoted string, it must be only one character. See the Key Value Specification topic for details. |
| Syntax error. | This error can be caused by punctuation errors. |
| System variable name"%s" reused. | The variable name %s is one of the predefined Flight Simulator variables or functions. %s represents the actual variable name. |
| THEN expected. | |
| TITLE too long (limit is %d characters). | The length limit is 63 bytes. |
| TO expected. | |
| Two ELSEs with same IF. | |
| Unbalanced quotation marks. | |
| Unknown key name "%s". | The KV( ) function doesn't recognize %s as a key name. %s represents the actual string. |
| Unrecognized command "%s". | %s represents the unrecognized command name. Check the spelling. |
| Unrecognized keyword "%s". | %s represents the unrecognized keyword. Check the spelling. |
| Undeclared variable "%s". | %s represents a variable that is not predefined and has not been declared. Variables must be declared before they are used. Check the spelling. |
| Variable name expected. | The compiler failed to find a variable name. This error can be caused by a punctuation error or an ill-formed variable name. Variable names must start with a letter and can contain only letters, digits, and underscores. |
| WHILE at line %d has no ENDWHILE. | |
| "(" expected. | |
| ")" expected. | |
| "," expected. | |
| "=" expected. | |
| TRUE or FALSE are | |
| reserved keywords. | TRUE and FALSE are built-in constants and can't be declared as variables. |