diff --git a/core/src/main/kotlin/timetables/dto/TimetableThemeDto.kt b/core/src/main/kotlin/timetables/dto/TimetableThemeDto.kt
index 88105407..396930a6 100644
--- a/core/src/main/kotlin/timetables/dto/TimetableThemeDto.kt
+++ b/core/src/main/kotlin/timetables/dto/TimetableThemeDto.kt
@@ -4,6 +4,7 @@ import com.wafflestudio.snu4t.common.enum.BasicThemeType
 import com.wafflestudio.snu4t.timetables.data.ColorSet
 import com.wafflestudio.snu4t.timetables.data.TimetableTheme
 import com.wafflestudio.snu4t.timetables.service.toBasicThemeType
+import com.wafflestudio.snu4t.timetables.service.toIdForTimetable
 
 data class TimetableThemeDto(
     val id: String?,
@@ -17,7 +18,7 @@ data class TimetableThemeDto(
 fun TimetableThemeDto(timetableTheme: TimetableTheme) =
     with(timetableTheme) {
         TimetableThemeDto(
-            id = if (isCustom) id else null,
+            id = toIdForTimetable(),
             theme = toBasicThemeType(),
             name = name,
             colors = colors,
diff --git a/core/src/main/kotlin/timetables/service/TimetableService.kt b/core/src/main/kotlin/timetables/service/TimetableService.kt
index eb5e51a2..8a117845 100644
--- a/core/src/main/kotlin/timetables/service/TimetableService.kt
+++ b/core/src/main/kotlin/timetables/service/TimetableService.kt
@@ -69,7 +69,7 @@ class TimetableServiceImpl(
             semester = timetableRequest.semester,
             title = timetableRequest.title,
             theme = defaultTheme.toBasicThemeType(),
-            themeId = defaultTheme?.id,
+            themeId = defaultTheme.toIdForTimetable(),
             isPrimary = timetableRepository
                 .findAllByUserIdAndYearAndSemester(userId, timetableRequest.year, timetableRequest.semester)
                 .toList()
@@ -122,7 +122,7 @@ class TimetableServiceImpl(
         val theme = timetableThemeService.getTheme(userId, themeId, basicThemeType)
 
         timetable.theme = theme.toBasicThemeType()
-        timetable.themeId = theme.id
+        timetable.themeId = theme.toIdForTimetable()
 
         val colorCount = if (theme.isCustom) requireNotNull(theme.colors).size else BasicThemeType.COLOR_COUNT
         timetable.lectures.forEachIndexed { index, lecture ->
@@ -161,7 +161,7 @@ class TimetableServiceImpl(
             semester = coursebook.semester,
             title = "나의 시간표",
             theme = defaultTheme.toBasicThemeType(),
-            themeId = defaultTheme?.id,
+            themeId = defaultTheme.toIdForTimetable(),
         )
         timetableRepository.save(timetable)
     }
diff --git a/core/src/main/kotlin/timetables/service/TimetableThemeService.kt b/core/src/main/kotlin/timetables/service/TimetableThemeService.kt
index 589d840f..e557be6f 100644
--- a/core/src/main/kotlin/timetables/service/TimetableThemeService.kt
+++ b/core/src/main/kotlin/timetables/service/TimetableThemeService.kt
@@ -238,8 +238,7 @@ class TimetableThemeServiceImpl(
             val theme = getTheme(timetable.userId, timetable.themeId)
 
             val alreadyUsedColors = timetable.lectures.map { requireNotNull(it.color) }
-            val colors = requireNotNull(theme.colors) { "theme.colors 가 null - userId: ${timetable.userId}, timetableId: ${timetable.id}, themeId: ${theme.id}" }
-            val colorToCount = colors.associateWith { color -> alreadyUsedColors.count { it == color } }
+            val colorToCount = requireNotNull(theme.colors).associateWith { color -> alreadyUsedColors.count { it == color } }
 
             val minCount = colorToCount.minOf { it.value }
             0 to colorToCount.entries.filter { (_, count) -> count == minCount }.map { it.key }.random()
@@ -248,3 +247,4 @@ class TimetableThemeServiceImpl(
 }
 
 fun TimetableTheme?.toBasicThemeType() = if (this == null || isCustom) BasicThemeType.SNUTT else requireNotNull(BasicThemeType.from(name))
+fun TimetableTheme?.toIdForTimetable() = if (this == null || isCustom.not()) null else id