diff --git a/R/utils.R b/R/utils.R
index 13ee6205..6d01c23e 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -96,7 +96,12 @@ split_on_line_directives <- function(lines) {
 
   file_starts <- directive_lines + 1
   file_ends <- c(directive_lines[-1] - 1, length(lines))
-  res <- mapply(function(start, end) lines[start:end], file_starts, file_ends)
+  res <- mapply(
+    function(start, end) lines[start:end],
+    file_starts,
+    file_ends,
+    SIMPLIFY = FALSE
+  )
   names(res) <- na.omit(matches$filename)
   res
 }
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
index 8ee62041..90dc0a37 100644
--- a/tests/testthat/test-utils.R
+++ b/tests/testthat/test-utils.R
@@ -72,3 +72,34 @@ test_that("split_on_line_directives returns NULL for input without directive (#5
     NULL
   )
 })
+
+test_that("split_on_line_directives does not simplify the result (#588)", {
+  expect_identical(
+    split_on_line_directives(
+      c(
+        '#line 1 "foo.R"',
+        "abc",
+        "def"
+      )
+    ),
+    list(
+      "foo.R" = c("abc", "def")
+    )
+  )
+  expect_identical(
+    split_on_line_directives(
+      c(
+        '#line 1 "foo.R"',
+        "abc",
+        "def",
+        '#line 4 "bar.R"',
+        "ghi",
+        "jkl"
+      )
+    ),
+    list(
+      "foo.R" = c("abc", "def"),
+      "bar.R" = c("ghi", "jkl")
+    )
+  )
+})