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Error evaluating function `darken`: undefined is not a function

When Gulp compiles, I get this error:

error evaluating function `darken`: undefined is not a function in file /path_to_theme/bower_components/bootstrap/less/variables.less line no. 36
error evaluating function `darken`: undefined is not a function in file /path_to_theme/bower_components/bootstrap/less/variables.less line no. 36

The variables.less file is the default one, that is:

//
// Variables
// --------------------------------------------------


//== Colors
//
//## Gray and brand colors for use across Bootstrap.

@gray-base:              #000;
@gray-darker:            lighten(@gray-base, 13.5%); // #222
@gray-dark:              lighten(@gray-base, 20%);   // #333
@gray:                   lighten(@gray-base, 33.5%); // #555
@gray-light:             lighten(@gray-base, 46.7%); // #777
@gray-lighter:           lighten(@gray-base, 93.5%); // #eee

@brand-primary:         darken(#428bca, 6.5%); // #337ab7
@brand-success:         #5cb85c;
@brand-info:            #5bc0de;
@brand-warning:         #f0ad4e;
@brand-danger:          #d9534f;


//== Scaffolding
//
//## Settings for some of the most global styles.

//** Background color for `<body>`.
@body-bg:               #fff;
//** Global text color on `<body>`.
@text-color:            @gray-dark;

//** Global textual link color.
@link-color:            @brand-primary;
//** Link hover color set via `darken()` function.
@link-hover-color:      darken(@link-color, 15%);
//** Link hover decoration.
@link-hover-decoration: underline;

    ...

Sorry, that was my error: I overrides the brand-primary variable with a wrong color hex (missing the #)