diff --git a/kmath-functions/src/commonMain/kotlin/scientifik/kmath/interpolation/LinearInterpolator.kt b/kmath-functions/src/commonMain/kotlin/scientifik/kmath/interpolation/LinearInterpolator.kt
index 9467da421..98beb4391 100644
--- a/kmath-functions/src/commonMain/kotlin/scientifik/kmath/interpolation/LinearInterpolator.kt
+++ b/kmath-functions/src/commonMain/kotlin/scientifik/kmath/interpolation/LinearInterpolator.kt
@@ -17,7 +17,7 @@ class LinearInterpolator<T : Comparable<T>>(override val algebra: Field<T>) : Po
         OrderedPiecewisePolynomial(points.x[0]).apply {
             for (i in 0 until points.size - 1) {
                 val slope = (points.y[i + 1] - points.y[i]) / (points.x[i + 1] - points.x[i])
-                val const = points.x[i] - slope * points.x[i]
+                val const = points.y[i] - slope * points.x[i]
                 val polynomial = Polynomial(const, slope)
                 putRight(points.x[i + 1], polynomial)
             }
diff --git a/kmath-functions/src/commonTest/kotlin/scientifik/kmath/interpolation/LinearInterpolatorTest.kt b/kmath-functions/src/commonTest/kotlin/scientifik/kmath/interpolation/LinearInterpolatorTest.kt
index 6d35d19f1..23acd835c 100644
--- a/kmath-functions/src/commonTest/kotlin/scientifik/kmath/interpolation/LinearInterpolatorTest.kt
+++ b/kmath-functions/src/commonTest/kotlin/scientifik/kmath/interpolation/LinearInterpolatorTest.kt
@@ -1,5 +1,6 @@
 package scientifik.kmath.interpolation
 
+import scientifik.kmath.functions.PiecewisePolynomial
 import scientifik.kmath.functions.asFunction
 import scientifik.kmath.operations.RealField
 import kotlin.test.Test
@@ -15,7 +16,7 @@ class LinearInterpolatorTest {
             2.0 to 3.0,
             3.0 to 4.0
         )
-        val polynomial = LinearInterpolator(RealField).interpolatePolynomials(data)
+        val polynomial: PiecewisePolynomial<Double> = LinearInterpolator(RealField).interpolatePolynomials(data)
         val function = polynomial.asFunction(RealField)
 
         assertEquals(null, function(-1.0))