diff --git a/lib/node_modules/@stdlib/math/base/special/acosf/lib/main.js b/lib/node_modules/@stdlib/math/base/special/acosf/lib/main.js
index 0fb76378ac31..1a8afefabefe 100644
--- a/lib/node_modules/@stdlib/math/base/special/acosf/lib/main.js
+++ b/lib/node_modules/@stdlib/math/base/special/acosf/lib/main.js
@@ -111,7 +111,7 @@ function acosf( x ) {
 		r = float64ToFloat32( p / q );
 		return float64ToFloat32( PIO2_HI - float64ToFloat32( x - float64ToFloat32( PIO2_LO - float64ToFloat32( x * r ) ) ) ); // eslint-disable-line max-len
 	}
-	if ( x < -0.5 ) {
+	if ( x <= -0.5 ) {
 		z = float64ToFloat32( 0.5 * float64ToFloat32( 1.0 + x ) );
 		p = float64ToFloat32( z * polyp( z ) );
 		q = float64ToFloat32( 1.0 + float64ToFloat32( z * QS1 ) );
diff --git a/lib/node_modules/@stdlib/math/base/special/acosf/test/test.js b/lib/node_modules/@stdlib/math/base/special/acosf/test/test.js
index 371b9914a37d..aa74e1d3f16b 100644
--- a/lib/node_modules/@stdlib/math/base/special/acosf/test/test.js
+++ b/lib/node_modules/@stdlib/math/base/special/acosf/test/test.js
@@ -140,6 +140,12 @@ tape( 'the function returns `PI` if provided a value equal to `-1`', function te
 	t.end();
 });
 
+tape( 'the function returns `2π/3` if provided `-0.5`', function test( t ) {
+	var v = acosf( -0.5 );
+	t.strictEqual( v, float64ToFloat32( 2.0 * PI / 3.0 ), 'returns expected value' );
+	t.end();
+});
+
 tape( 'the function returns `0` if provided `1`', function test( t ) {
 	t.strictEqual( isPositiveZero( acosf( 1.0 ) ), true, 'returns expected value' );
 	t.end();
diff --git a/lib/node_modules/@stdlib/math/base/special/acosf/test/test.native.js b/lib/node_modules/@stdlib/math/base/special/acosf/test/test.native.js
index 013d6750b464..26b08194ce35 100644
--- a/lib/node_modules/@stdlib/math/base/special/acosf/test/test.native.js
+++ b/lib/node_modules/@stdlib/math/base/special/acosf/test/test.native.js
@@ -149,6 +149,12 @@ tape( 'the function returns `PI` if provided a value equal to `-1`', opts, funct
 	t.end();
 });
 
+tape( 'the function returns `2π/3` if provided `-0.5`', opts, function test( t ) {
+	var v = acosf( -0.5 );
+	t.strictEqual( v, float64ToFloat32( 2.0 * PI / 3.0 ), 'returns expected value' );
+	t.end();
+});
+
 tape( 'the function returns `0` if provided `1`', opts, function test( t ) {
 	t.strictEqual( isPositiveZero( acosf( 1.0 ) ), true, 'returns expected value' );
 	t.end();