diff --git a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/cumSum.kt b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/cumSum.kt
index b32409c692..cede3a6932 100644
--- a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/cumSum.kt
+++ b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/cumSum.kt
@@ -24,15 +24,19 @@ import kotlin.reflect.KProperty
  * from the first cell to the last cell.
  *
  * __NOTE:__ If the column contains nullable values and [skipNA\] is set to `true`,
- * null and NaN values are skipped when computing the cumulative sum.
- * When false, all values after the first NA will be NaN (for Double and Float columns)
- * or null (for integer columns).
+ * `null` and `NaN` values are skipped when computing the cumulative sum.
+ * When `false`, all values after the first `NA` will be `NaN` (for [Double] and [Float] columns)
+ * or `null` (for other columns).
  *
- * {@get [CumSumDocs.CUMSUM_PARAM] @param [columns\]
- * The names of the columns to apply cumSum operation.}
+ * `cumSum` only works on columns that contain solely primitive numbers.
  *
- * @param [skipNA\] Whether to skip null and NaN values (default: `true`).
+ * Similar to [sum][sum], [Byte][Byte]- and [Short][Short]-columns are converted to [Int][Int].
  *
+ * {@get [CumSumDocs.CUMSUM_PARAM] @param [columns\] The selection of the columns to apply the `cumSum` operation to.
+ *   If not provided, `cumSum` will be applied to all primitive columns [at any depth][ColumnsSelectionDsl.colsAtAnyDepth].
+ * }
+ *
+ * @param [skipNA\] Whether to skip `null` and `NaN` values (default: `true`).
  * @return A new {@get [CumSumDocs.DATA_TYPE]} of the same type with the cumulative sums.
  *
  * {@get [CumSumDocs.CUMSUM_PARAM] @see [Selecting Columns][SelectSelectingOptions].}
@@ -41,8 +45,11 @@ import kotlin.reflect.KProperty
 @ExcludeFromSources
 @Suppress("ClassName")
 private interface CumSumDocs {
+
+    // Can be emptied to disable information about selecting columns
     interface CUMSUM_PARAM
 
+    // Either [DataColumn] or [DataFrame]
     interface DATA_TYPE
 }
 
@@ -157,10 +164,11 @@ public fun <T> DataFrame<T>.cumSum(
  * {@set [CumSumDocs.DATA_TYPE] [DataFrame]}
  * {@set [CumSumDocs.CUMSUM_PARAM]}
  */
+@Refine
+@Interpretable("DataFrameCumSum0")
 public fun <T> DataFrame<T>.cumSum(skipNA: Boolean = defaultCumSumSkipNA): DataFrame<T> =
     cumSum(skipNA) {
-        // TODO keep at any depth?
-        colsAtAnyDepth().filter { it.isNumber() }.cast()
+        colsAtAnyDepth().filter { it.isPrimitiveOrMixedNumber() }.cast()
     }
 
 // endregion
@@ -212,10 +220,11 @@ public fun <T, G> GroupBy<T, G>.cumSum(
  * {@set [CumSumDocs.DATA_TYPE] [GroupBy]}
  * {@set [CumSumDocs.CUMSUM_PARAM]}
  */
+@Refine
+@Interpretable("GroupByCumSum0")
 public fun <T, G> GroupBy<T, G>.cumSum(skipNA: Boolean = defaultCumSumSkipNA): GroupBy<T, G> =
     cumSum(skipNA) {
-        // TODO keep at any depth?
-        colsAtAnyDepth().filter { it.isNumber() }.cast()
+        colsAtAnyDepth().filter { it.isPrimitiveOrMixedNumber() }.cast()
     }
 
 // endregion
diff --git a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/math/cumsum.kt b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/math/cumsum.kt
index 6f73220b66..2e7c6b505b 100644
--- a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/math/cumsum.kt
+++ b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/math/cumsum.kt
@@ -292,7 +292,7 @@ internal fun DataColumn<Long?>.cumSumImpl(skipNA: Boolean): DataColumn<Long?> {
  * T : Number(?) -> T(?)
  */
 public val cumSumTypeConversion: CalculateReturnType = { type, _ ->
-    when (val type = type.withNullability(false)) {
+    when (type.withNullability(false)) {
         // type changes to Int, carrying nullability
         typeOf<Short>(), typeOf<Byte>() -> typeOf<Int>().withNullability(type.isMarkedNullable)
 
diff --git a/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/statistics/cumsum.kt b/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/statistics/cumsum.kt
index 05fc9438ce..04a44bbe10 100644
--- a/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/statistics/cumsum.kt
+++ b/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/statistics/cumsum.kt
@@ -1,5 +1,6 @@
 package org.jetbrains.kotlinx.dataframe.statistics
 
+import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.matchers.shouldBe
 import org.jetbrains.kotlinx.dataframe.DataColumn
 import org.jetbrains.kotlinx.dataframe.api.columnOf
@@ -8,7 +9,10 @@ import org.jetbrains.kotlinx.dataframe.api.cumSum
 import org.jetbrains.kotlinx.dataframe.api.dataFrameOf
 import org.jetbrains.kotlinx.dataframe.api.groupBy
 import org.jetbrains.kotlinx.dataframe.api.map
+import org.jetbrains.kotlinx.dataframe.impl.nullableNothingType
+import org.jetbrains.kotlinx.dataframe.math.cumSumTypeConversion
 import org.junit.Test
+import kotlin.reflect.typeOf
 
 @Suppress("ktlint:standard:argument-list-wrapping")
 class CumsumTests {
@@ -92,4 +96,39 @@ class CumsumTests {
                 "c", 4,
             )
     }
+
+    @Test
+    fun `df cumSum default`() {
+        val df = dataFrameOf(
+            "doubles" to columnOf(1.0, 2.0, null),
+            "shorts" to columnOf(1.toShort(), 2.toShort(), null),
+            "bigInts" to columnOf(1.toBigInteger(), 2.toBigInteger(), null),
+            "mixed" to columnOf<Number?>(1.0, 2, null),
+        )
+
+        val res = df.cumSum()
+
+        // works for Doubles, turns nulls into NaNs
+        res["doubles"].values() shouldBe columnOf(1.0, 3.0, Double.NaN).values()
+        // works for Shorts, turns into Ints, skips nulls
+        res["shorts"].values() shouldBe columnOf(1, 3, null).values()
+        // does not work for big numbers, keeps them as is
+        res["bigInts"].values() shouldBe columnOf(1.toBigInteger(), 2.toBigInteger(), null).values()
+        // works for mixed columns of primitives, number-unifies them; in this case to Doubles
+        res["mixed"].values() shouldBe columnOf(1.0, 3.0, Double.NaN).values()
+    }
+
+    @Test
+    fun `cumSumTypeConversion tests`() {
+        cumSumTypeConversion(typeOf<Int>(), false) shouldBe typeOf<Int>()
+        cumSumTypeConversion(typeOf<Long?>(), false) shouldBe typeOf<Long?>()
+        cumSumTypeConversion(typeOf<Short?>(), false) shouldBe typeOf<Int?>()
+        cumSumTypeConversion(typeOf<Byte>(), false) shouldBe typeOf<Int>()
+        cumSumTypeConversion(typeOf<Float?>(), false) shouldBe typeOf<Float>()
+        cumSumTypeConversion(typeOf<Double?>(), false) shouldBe typeOf<Double>()
+        cumSumTypeConversion(typeOf<Double>(), false) shouldBe typeOf<Double>()
+        cumSumTypeConversion(nullableNothingType, false) shouldBe nullableNothingType
+
+        shouldThrow<IllegalStateException> { cumSumTypeConversion(typeOf<String>(), false) }
+    }
 }