diff --git a/cpu/stm32_common/periph/i2c_1.c b/cpu/stm32_common/periph/i2c_1.c
index 61d8347155..084e0685dc 100644
--- a/cpu/stm32_common/periph/i2c_1.c
+++ b/cpu/stm32_common/periph/i2c_1.c
@@ -32,6 +32,7 @@
  * @}
  */
 
+#include <assert.h>
 #include <stdint.h>
 #include <errno.h>
 
@@ -145,14 +146,13 @@ int i2c_acquire(i2c_t dev)
     return 0;
 }
 
-int i2c_release(i2c_t dev)
+void i2c_release(i2c_t dev)
 {
     assert(dev < I2C_NUMOF);
 
     periph_clk_dis(i2c_config[dev].bus, i2c_config[dev].rcc_mask);
 
     mutex_unlock(&locks[dev]);
-    return 0;
 }
 
 int i2c_write_regs(i2c_t dev, uint16_t addr, uint16_t reg,
diff --git a/cpu/stm32_common/periph/i2c_2.c b/cpu/stm32_common/periph/i2c_2.c
index 7c1b38ddef..8a1542417b 100644
--- a/cpu/stm32_common/periph/i2c_2.c
+++ b/cpu/stm32_common/periph/i2c_2.c
@@ -34,6 +34,7 @@
  * @}
  */
 
+#include <assert.h>
 #include <stdint.h>
 #include <errno.h>
 
@@ -195,7 +196,7 @@ int i2c_acquire(i2c_t dev)
     return 0;
 }
 
-int i2c_release(i2c_t dev)
+void i2c_release(i2c_t dev)
 {
     assert(dev < I2C_NUMOF);
 
@@ -207,7 +208,6 @@ int i2c_release(i2c_t dev)
 #endif
 
     mutex_unlock(&locks[dev]);
-    return 0;
 }
 
 int i2c_read_bytes(i2c_t dev, uint16_t address, void *data, size_t length,